Stove method: Place the chopped chocolate in a heat-proof bowl. Place the heavy cream in a small pot and heat over medium heat until the heavy cream comes to a simmer (small bubbles appear on the sides of the pot). Pour the hot cream onto the chocolate and let it stand for 1 minute. Stir the mixture until all the chocolate has melted and the mixture is smooth. Add in the butter and stir until all the butter has melted. Let the mixture cool at room temperature until the ganache has thickened. You can place it in the fridge to speed up the process.
Microwave method: Place the heavy cream in a microwave-safe bowl (glass or ceramic). Microwave the cream on medium for 30 seconds until the cream is hot. Add in the chopped chocolate and stir - the chocolate will start to melt. Place the bowl back into the microwave and microwave for 10 seconds. Stir the mixture and if there's any remaining un-melted chocolate, repeat the process (do not microwave for too long at one go unless you want to burn your ganache). Once all the chocolate has melted, add in the butter and let the mixture cool.
If making the ganache a day in advance, cover and refrigerate it. Take it out of the fridge 15 - 30 minutes to let it soften before filling the macarons.
Sift the almond flour, icing sugar and cocoa powder in a large bowl. Alternatively, place it 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 almond flour, icing sugar and cocoa powder all at once. Using a large rubber spatula, fold in the dry ingredients until no streaks of dry ingredients remained. Do 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.
Let the macarons rest for 45 minutes (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 be 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 heaping teaspoon of chocolate ganache (more if desired) in the middle of a macaron. Sandwich with the other macaron - gently twisting and pressing it down until the ganache 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 ganache 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.