Stir together the zest, lemon juice and sugar in a small pot. Bring to a simmer over low heat. Place the egg yolks in a clean bowl. Whisk it gently with a wire whisk. Temper the egg yolks and return the egg yolks mixture to the pot. Cook over medium-low heat until the liquid has thickened, which will take about 5 minutes. Remove from heat and let it cool completely, stirring frequently.
Transfer the lemon curd to a bowl or a container. Cover the top directly with clingfilm (press the clingfilm down so that it touches the surface of the curd) and refrigerated until needed.
Macarons
Sift the almond flour and icing sugar in a large bowl. Alternatively, place them in a food processor and blitz for several seconds to break up any lumps. It is essential to remove the lumps so that there will not be any bumps on top of the macarons.
In a clean, dry, oil-free bowl, add in the egg whites. Using an electric mixer, turn the speed to low and whisk the egg whites for 10 seconds. Increase the speed up to high and whisk for another 30 seconds, until the egg whites and foamy and white in colour. Add in the caster sugar - 1 tablespoon at a time - and whisk for 30 seconds (on high speed) before adding the next tablespoon.
Continue whisking on high speed until the meringue is white, shiny and forms stiff peaks. To check for stiff peaks, turn off the electric mixer and lift it up and turn it upside down. If the meringue remains vertical and did not droop down, the meringue is ready. If not, continue whisking and check frequently - you don't want to overwhisk the egg whites.
Once the meringue is ready, add in the sifted almond flour, icing sugar and lemon zest all at once. Add in a little food colouring. Using a large rubber spatula, fold in the dry ingredients until no streaks of dry ingredients remained. Scrape the sides frequently to incorporate all the dry ingredients. The batter should be shiny, sticky, thick but liquid (this is what they like to call "lava" like batter): scoop up some batter and drop some it on to itself, the dropped batter should slowly sink and merge back with the remaining batter in more than 5 seconds but less than 20-30 seconds. If it sinks and merge back within 5 seconds, it is likely that you have over-mixed the batter. If it doesn't sink back, fold a few more times, and test again. It is a tricky balance - you can't under-fold and you can't over-fold either.
Line several baking sheets with baking parchment. Transfer the batter into a large piping bag with a plain tip. The batter should flow out easily. Pipe small rounds of batter (about 1-inch in diameter) onto the baking sheets, leaving a 1.5-inch spacing in between.
After piping, bang the sheets on the table counter to release air bubbles and to flatten the shells. Use a tooth-pick to poke the air bubbles (especially the big ones) and a small spatula to smooth out the tops. This step is optional but it does help to give you prettier macarons.
Optional: to make the heart shape on the macarons, reserve 1 tablespoon of macaron batter and colour it with red food colouring. Pipe 5 macarons and use a toothpick to draw the heart with the red macaron batter. Alternatively, spoon the red macaron batter in a small piping bag and pipe a small heart. Bang the sheets on the table counter to release the air bubbles. Proceed with the steps below. Don't pipe too many macarons at one time as they will dry out, making it hard to draw the heart.
Let the macarons rest for 45 minutes (the weather in Singapore is humid, hence I let them rest longer, if the weather is dry, you'll only need to rest them for about 20 to 30 minutes). Touch them gently (top and the sides) - they should not be tacky / sticky and they will not look shiny anymore.
Preheat the oven to 150 degrees Celsius.
Bake the macarons, one sheet at a time, for 10 minutes, then rotate the sheets 180 degrees and bake for another 8 minutes until the top is crisp and the feet do not wobble - touch the top gently and if you see the feet shaking, they are not ready yet - bake and check every 1 minute.
Place the baking sheet on a wire rack. Let the macarons cool for 5 minutes. Slide the parchment paper of macrons onto a damp tea towel. The thermal shock (hot from the macarons and cold from the damp tea towel) will help to release the macarons from the baking parchment. Remove the macarons from the baking parchment and cool completely on a wire rack.
Pair the macarons so that the same size ones are matched together. Place a small teaspoon of lemon curd (a little goes a long way) in the middle of a macaron. Sandwich with the other macaron - gently twisting and pressing it down until the lemon curd extends to the edge of the macaron. Do not eat the macarons immediately!
Place the macarons in a container and refrigerate them for 24 hours to help the flavours meld together - the lemon curd will help soften the macaron to achieve the crispy exterior, chewy interior texture. Take them out of the fridge 15 minutes before serving so that they will be at room temperature.