Sift together almond flour and confectioners sugar into a large bowl and whisk to remove lumps.

Heat egg whites, white sugar, and egg white powder in a heatproof bowl over simmering water until 140°F, then whip until stiff peaks.

Gently fold the dry ingredients into the meringue until smooth and slightly runny, resembling thick lava.

Pipe batter onto parchment-lined baking sheet in 1.5-inch circles and sprinkle with nonpareils.

Let piped macarons dry at room temperature for 30-60 minutes, then bake at 300°F for 15-20 minutes.

Prepare buttercream by heat-treating cake mix, then cream with butter, confections sugar, heavy cream, and vanilla.

Pipe buttercream onto half of the macaron shells and sandwich with the other half. Refrigerate overnight for best flavor.