Dry Ingredients:
Prepare a baking sheet with parchment or silicone mat (prepare 2 sheets if possible).
Sift almond flour and confectioners sugar together in a large bowl and whisk until combined. Set aside.
Making the Meringue:
In the stand mixer bowl, combine egg whites, white sugar, and egg white powder.
Place the bowl over simmering water (bain-marie), whisking until the mixture reaches 140°F (60°C).
Remove from heat and beat the mixture on low speed, gradually increasing to high, until stiff peaks form. Add vanilla and beat until just combined.
Macaronage:
Sift the dry ingredients into the meringue.
Gently fold until the mixture ribbons off the spatula. The batter should be smooth, forming figure 8 shapes without breaking.
Piping and Drying the Macarons:
Pipe small circles (1½ inch diameter) on the prepared baking sheets.
Tap sheets to release air bubbles, sprinkle with rainbow sprinkles, and allow to sit until a skin forms, about 8 minutes to 1 hour.
Baking:
Preheat oven to 300°F (or 275°F for convection).
Bake for 15-20 minutes. Allow to cool before removing from the sheets. Pair matching macaron halves.
Birthday Cake Buttercream:
Microwave the boxed cake mix for 1 minute 30 seconds, stirring every 30 seconds.
In the stand mixer, beat butter until fluffy. Add cooled cake mix and confectioners sugar in intervals until smooth.
Add heavy cream, vanilla, and food coloring. Pipe the buttercream onto one macaron half and top with the other.
Refrigerate overnight to mature.