A day before you prepare the macarons, separate the eggs and leave 220g of egg whites on the counter in a small bowl covered with plastic wrap. This will allow the eggs to become more liquid and make a better macaron batter.
Prepare a template for the macarons by using a small glass measuring about 3.5 cm to trace circles onto craft paper or parchment paper that are 2 cm apart. You can keep this template for future use.
Place the powdered sugar and ground almonds in a food processor and blend for a 1-2 minutes.
Pass the blended ground almonds and sugar through a sieve and replace all the pieces of almonds that were not ground with same quantity of finer ground.
Place the powedered sugar-almond mixture in a large bowl and add 110 g aged egg whites to that bowl. Do not mix in the egg whites. Reserve.
Place the remaining 110g egg whites in a stand mixer.
Bring the 300 g regular sugar and the 75 g of mineral water to a boil in a small saucepan with a spout preferably.
Once the sugar as reached 115C, begin whipping the egg whites at high speed to soft peaks.
When the sugar reaches 118C, pour the sugar into the egg whites. The mixture will quickly become glossy. Continue beating for 3 minutes.
Let the whipped egg whites temperature decrease to 50C. At that point, incorporate the egg whites into the almond-powedered sugar mixture. Add the melted chocolate in at this time as well.
With a spatula, mix the batter gently for about 3-5 minutes. You want the batter to go down in volume to make sure there is not too much air left in.
Preheat the oven to 350F.
Insert a round piping tip measuring 2 cm into a cloth pastry bag.
Place the template onto a cookie sheet underneath parchment paper.
Holding the pastry bag about an inch above the parchment paper, squeeze the pastry bag until the batter is almost at the line of the circle. The batter spreads slightly and so you don't add too much.
Hold the cookie sheet carefully and tap underneath the tray to make sure any trapped air bubbles in the rounds can escape. Continue with the rest of the batter and allow to rest for about 30 minutes on the counter.
Bake the macarons for 11 minutes, opening the oven door 2 times to allow air in.
If you see your macarons have browned, your oven may be strong meaning you should take the macarons out after 10 minutes..
Let the macarons cool on a rack until ready to be filled.
To prepare the ganache, cut the butter into small cubes, and roughly chop the chocolates. Bring the cream to a boil in a small saucepan and pour over the chocolate. Mix until the chocolate has melted thoroughly. Once the ganache temperature has dropped to 50C, you can incorporate the butter pieces. Place the ganache into a container and cover ganache surface diretcly with plastic wrap. Allow to stay in fridge until the chocolate becomes creamy.
Match the cookies up before filling to make sure top and bottom are of similar size.
Using a pastry bag and same piping tip size as earlier, fill the cookie generously with ganache.
Macarons are best after they have stayed in the refrigerator for 24 hours, since some kind of magical osmosis takes place between the cookie and the ganache, making the whole just divinely moist.
Bring to room temperature at least 2 hours before serving.