2 On another baking sheet, spread cashews in an even layer. Roast in the preheated oven for 30 minutes. Allow to cool.
3 In a food processor, combine almond meal, cashews, and confectioners’ sugar. Process for 1 minute until very finely ground, stopping every 15 seconds to scrape down the mixture with a spatula to ensure that the mixture does not clump up.
4 Place egg whites in the bowl of an electric mixer and whip until soft peaks form. Start slowly and then build up speed.
5 While egg whites are being beaten, add caster sugar 50 grams at a time.
6 Beat egg whites until stiff peaks form. Add a few drops food coloring. (Refrain from adding food coloring until after you've whipped the egg whites to the stiff peaks stage—any earlier and the stiff peaks might not form.)
7 Fold the ground almond and cashew mixture into the egg white mixture in three additions until fully combined.
8 Preheat oven to 300°F.
9 Pipe mixture onto prepared baking tray. Set tray aside for around 30 minutes to let the shells dry. Bake for 13 to 15 minutes.
10 Prepare the buttercream: Place sugar in a saucepan, add water and make sure that sugar is dampened. Set over medium heat and bring to a boil; do not stir. Place a candy thermometer in the boiling syrup. Bring temperature to 240°F (firm ball stage).
11 While sugar is cooking, place yolks in the bowl of an electric mixer fitted with the whisk attachment. Beat at medium speed then increase to high until yolks are pale and foamy.
12 Once syrup reaches 240°F, turn off heat. Let the bubbling subside for 30 seconds and pour syrup in a thin and steady stream over egg yolks while the mixer is running at high speed. When all the syrup has been absorbed, continue to beat until the mixture cools to room temperature.
13 On medium-low speed, add softened butter a few cubes at a time, waiting for each addition to be incorporated before adding the next. After all the butter has been added, increase speed to medium, and continue whipping until mixture is thick and smooth, about 5 minutes.
14 Add cashews and vanilla; mix on high for another 2 minutes until fully incorporated.
15 Allow to cool slightly before piping onto macaron shells.
Egg white tip: Keep egg whites in a container with the lid half open (or a lid with holes) for at least a day before use. This will allow the water from the whites to evaporate. As a result, whipped egg whites will be stiffer and stronger, and you can ensure they won’t deflate easily during the folding process.