Sift together 1 cup of powdered sugar and ½ cup of almond flour in a bowl twice and set aside.

In a mixing bowl, beat 3 egg whites until frothy, gradually add ¼ cup sugar and 1/8 teaspoon cream of tartar until stiff peaks form.

Add green gel food coloring to the whipped egg whites, then gently fold in the sifted almond flour and powdered sugar mixture until smooth.

Pipe the macaron batter onto parchment-lined baking sheets in desired shapes. Tap to release air bubbles.

Let the piped macarons sit at room temperature for 30-60 minutes until a skin forms.

Preheat the oven to 300°F (150°C). Bake for 14-16 minutes, rotate pans halfway, and allow to cool completely.

Beat ¼ cup butter until fluffy, mix in powdered sugar, vanilla extract, heavy cream, and cake crumbles until smooth.

Assemble macarons by piping filling onto one shell and topping it with another shell.