Place the icing sugar, cocoa powder and almond flour in a food processer and blitz for 2-3 minutes. Transfer to a fine mesh sieve and gently sift the mixture into a bowl. Discard any pieces of almond flour that do not pass through the sieve. If you are discarding a lot of almond flour weigh the amount that is being discarded and replace with the same amount of finely sifted almond flour.
Place egg whites the bowl that was cleaned with vinegar/lemon juice and begin to whip on low speed using your electric mixer.
Once the eggs start to look frothy, add the cream of tartar.
Continue whipping until the eggs look foamy, then gradually add the caster sugar, 1 tablespoon at a time, mixing well after each addition until all the sugar has been incorporated.
Turn the mixer up to medium-high and continue to whip the meringue until it reaches stiff peaks.
Double check that all the sugar has been dissolved by carefully rubbing some of the meringue between your fingers. If you feel any grains of sugar continue whipping the meringue and test again. Repeat this until the meringue is smooth to touch.
Once the meringue is ready it is time to combine it with the almond flour/icing sugar. You want to do this while keeping as much air in the meringue as possible. Best method is to add half the almond flour to your meringue and gently fold in - aka use your spatula to scrape around the outside of the bowl until this first batch of almond flour has been mixed in). Then, fold in remaining almond flour using the same technique.
Now it's time to perform the macaronage. In this stage we are trying to remove some of the air from the meringue. To do this use your spatula to gently spread the batter around the sides of the bowl, pressing it slightly as you do so. Continue doing this until your batter reaches the consistency of hot lava. Check out my blog post on how to make the perfect macarons at home for more information about how to perfect the macaronage. Once your batter is at hot lava stage, transfer it to your prepared piping bag.
Place a dap of macaron batter in between the baking paper and tray in each corner to hold the baking paper in place, then pipe batter onto your prepared baking tray.
Bang the tray on your counter a couple of times to bring and any air bubbles to the surface of the macarons, then use a toothpick to pop any visible bubbles and smooth the top of the cookie.
Rest the macaron cookies by leaving the trays on the bench until the tops of the macarons feel dry to touch. The amount of time this takes will depend on the weather in your area. Where I am in Melbourne, it takes about 20mins. You'll know the macarons are ready to bake if they feel smooth and dry to touch.
Preheat oven to 110 degrees. Transfer the tray of macarons to the oven. Bake at 110 degrees Celsius for 18 minutes (or a little longer if you are using a silicone mat instead of baking paper). You will know when the macarons are cooked because they will peel easily off the baking paper. Leave the macarons on the baking tray until they have cooled completely.