Divide the egg whites equally into 2 bowls. Set aside one bowl. In the other bowl, sift the almond meal, powdered sugar and matcha powder. Mix with a rubber spatula until you get a thick almond paste.
Add the water and granulated sugar to a small saucepan and set on medium heat. When the sugar syrup reaches 110°C (230F) start beating the egg whites from the second bowl. When the syrup reaches 121°C (250F) pour it in one thin stream into the beaten egg whites and keep mixing until the meringue cools to room temperature.
When the meringue has cooled, add the almond paste to the meringue bowl in two batches. Work it with a spatula gently and firmly at the same time, pressing the spatula against the sides of the bowl. This helps to get rid of excess air in the batter. Stop as soon as you see that the mixture is smooth and clump free. See tips below.
Transfer the batter to a piping bag and pipe small circles onto a tray lined with parchment paper. The macarons need to be quite spaced out as they tend to grow larger as the batter sets. Let rest for 30 minutes, or until the surface of the macaron shells is dry to the touch.
Bake the macarons in the preheated oven at 145°C (293F) for 18 minutes. Remove the parchment paper sheet from the hot baking tray as soon as you take the shells out of the oven. Let the macarons cool completely before filling.
Add the room temperature butter to a bowl together with the powdered sugar (sifted) and the vanilla. Mix at high speed for 4-5 minutes, until fluffy and light in colour. Add buttercream to a piping bag and fill the macarons.
Notes
The yield should be about 50 macarons. If you'd rather only fill half of them, and save half of the shells for later, halve the amounts in the buttercream recipe.
Make sure you use finely ground almond meal, you should be able to sift it through a regular mesh sieve (not a fine one like for flour).
Although you can switch to US measurements in the recipe card, I strongly recommend to use a kitchen scale and follow the recipe in grams.
The eggs I have here in Sweden are medium-small, and in order to get the 150 g of egg whites required by the recipe I generally need to use 5 eggs. The volume of the egg whites is roughly 120 ml (½ cup).
The nutritional information shared here is obtained using an online calculator and therefore only to be used as a guideline.