Sift together the superfine almond flour and confectioners sugar in a bowl.

In a heatproof bowl over simmering water, combine egg whites, granulated sugar, and egg white powder. Warm to 140°F (60°C) while stirring.

Transfer to a mixer and whip until stiff peaks form.

Gently fold in sifted dry ingredients until batter flows smoothly.

Pipe 1½-inch circles onto a parchment-lined baking sheet and sprinkle with rainbow sprinkles.

Let piped macarons sit at room temperature for 8 minutes to 1 hour until they form a skin.

Preheat oven to 300°F and bake for 15-20 minutes.

Allow macarons to cool completely on a wire rack.

Heat-treat the boxed cake mix, then mix with unsalted butter, confectioners sugar, heavy cream, and food coloring.

Pipe buttercream onto half of the macaron shells and sandwich with the remaining shells. Refrigerate for at least 30 minutes before serving.