Place the zest, juice, blueberries, water and sugar in a pot over low heat. Stir the berries constantly until the berries have softened and are releasing juices, about 10 minutes.
Strain the blueberries into a clean pot to remove the lemon zest and blueberry skins (I did not strain mine, but simply remove the lemon zest).
Add in the butter and stir until all the butter has melted. Stir in the eggs gradually. Cook the blueberry mixture over low heat, stirring constantly until it is thick enough to coat the back of a spoon, which will take about 10 to 15 minutes.
Transfer the curd into a clean container, let cool completely, cover and refrigerate overnight to let it set.
Simple Boozy Syrup
Place the sugar and water in a small pot. Bring the mixture to a boil, swirling the pot to dissolve the sugar crystals. Boil for 2 - 3 minutes on high heat then remove the pot from heat. Let cool to room temperature and stir in the rum if using.
Cake
Preheat the oven to 170 degrees Celsius. Grease an 8-inch round tin and line the bottom with greaseproof paper.
Measure the flour, cocoa powder, baking powder and salt in a small bowl. Use a wire whisk and whisk the dry ingredients together until they are all well-mixed and evenly distributed.
Measure the dark chocolate and unsalted butter in a large heatproof bowl and melt them over a pot of simmering water, stirring occasionally -Note [1].
Remove the heatproof bowl from the pot. Add in the caster sugar and whisk gently – not all of the sugar will dissolve, it’s ok if the mixture is grainy. Let the mixture stand for 10 minutes to cool down slightly (the eggs will scramble if added to a hot mixture).
Crack in one egg into the chocolate mixture. Whisk the mixture (yes, same wire whisk!) for 30 seconds until it is fully incorporated. Whisk in the second egg, and then whisk in the vanilla extract.
Sift a third of flour mixture into the chocolate batter and folding gently using a large metal spoon or rubber spatula. until all the dry ingredients have been incorporated – Note [2].
Transfer the cake batter into the lined cake tin. Bake for 30 to 40 minutes until a skewer inserted into the centre of the cake comes out clean.
Cool the cake in the tin for 10 to 15 minutes, then turn it out onto a wire rack and let it cool completely.
Vanilla Meringue
Preheat the oven to 120 degrees Celsius. Trace a 7-inch circle onto a greaseproof paper. Place the pencil side down onto a baking sheet. Butter the top of the greaseproof paper lightly. Set aside.
Place the egg whites in a clean, dry and oil-free bowl. Use an electric mixer and whip the egg whites on medium speed until frothy, about 2 minutes. Turn up the speed to medium-high and add in the caster sugar, a tablespoon at a time, until the meringue is creamy, about 4-5 minutes. Beat in the vanilla extract on medium speed.
Pipe or spread a 7-inch circle on the greaseproof paper, using the template drawn earlier as a guideline.
Bake for 55 minutes - 1 hour until the meringue is biege in colour - the meringue will spread out slightly and it will be dry and crunchy on the outside but chewy on the inside. Open the oven door ajar and let the meringue cool completely.
Easy Chocolate Mousse
Melt the chocolate and let it cool to lukewarm. Whip the cream to soft peaks formed and fold in the melted chocolate.
The mousse should be made just before assembling the cake so that it will be soft and liquid-y enough to pour it into the mould.
First Assembly
Level the cake so that the top is flat. Slice the cake into two horizontally.
Set an 8-inch ring mould onto a board. Moisten both sides of one cake slice and place it in the ring mould.
Pour half of the mousse onto the cake layer. Spread the mousse all over the cake evenly with a spatula or a spoon. Pour in half of the blueberry curd and spread it evenly.
Place the vanilla meringue onto the blueberry curd and press down gently. Pour the remaining mousse over, followed by the blueberry curd.
Moisten both sides of the remaining cake slice and place it on top of the blueberry curd, pressing it down gently.
Wrap the cake with plastic wrap and freeze it overnight or up to 1 week.
Dark Chocolate Ganache Glaze and Second Assembly
Heat the cream in a pot until small bubbles start to appear on the sides (simmering stage). Pour the cream over the chopped chocolate and let it sit for 2 minutes. Stir until all the chocolate has melted. Stir in the golden syrup. Let it sit aside to cool for 30 minutes.
Alternatively, heat the cream in a microwave for 30 seconds on medium high. Add in the chocolate and microwave at 15 seconds interval, checking the mixture and stirring it at each interval until all the chocolate has melted. Stir in the golden syrup and let cool for 30 minutes.
Remove the cake from the freezer and unmould the cake. Place the cake on a wire rack set over a baking sheet. Pour the chocolate ganache all over the cake, spreading it out with a spatula, letting the excess drip onto the baking sheet.
Refrigerate the cake while you prepare the final assembly.
Chocolate Border and Final Assembly
Wipe a large table with damp cloth. Immediately line a big piece of plastic wrap on the damp table so that it will stick to the table. This is to help with the ease of clean up later.
Measure the height and length of the cake border desired using a long piece of baking parchment and cut it out, leaving an inch on the sides and the top for easy handling later on. Place it on the plastic wrap and hold it down with some paper weights if your baking parchment tend to roll up.
Melt the chocolate. Transfer the melted chocolate into a small piping bag. Snip of a small corner of the end of the piping bag. Pipe rounds all over the baking parchment - it doesn't have to be very neat and precise and perfect ovals - it's called being artistic! :p Once the cake border goes up onto the cake, nobody will notice the flaws you think you've made because it will already look grand and beautiful!
Leave the piped chocolate aside for 5-10 minutes until it's slightly set but not yet harden - the piped chocolate will not ooze when lifted and when you touched it there will be no trace of melted chocolate on your finger. I wouldn't recommend putting the baking parchment in the refrigerator because the chocolate may harden faster than you think and you will need to start over.
Take the cake out of the fridge. Lift up the baking parchment of piped chocolate and wrap it carefully around the cake. Refrigerate until the chocolate has fully set and hardened. Discard the plastic wrap.
Peel off the baking parchment as gingerly as you can. If you leave the cake slightly longer at room temperature, the chocolate border will softened as curl downwards. I didn't expect for this to happen but nature took its course and I find that the cake looks great all the same.
For the leftover melted chocolate, piped wordings or small pearls at the bottom of the cake for a decorative finish.
Keep the cake in the refrigerator overnight to let it thaw out completely.