Prepare the dry ingredients by sifting together the powdered sugar and almond flour in a bowl. Set aside.

Whip egg whites with an electric mixer until foamy, then gradually add cream of tartar and granulated sugar until stiff peaks form.

Add green gel food coloring and gently fold into the meringue, then gradually add the sifted almond flour mixture.

Transfer macaron batter into a piping bag and pipe tree shapes on lined baking sheets.

Allow piped shells to rest at room temperature for 30-60 minutes.

Preheat oven to 300°F (150°C) and bake shells for 14-16 minutes.

Cool the shells completely on the baking mat or parchment.

Make the filling by beating the unsalted butter until light and fluffy, then mix in powdered sugar, heavy cream, vanilla extract, and cake crumbs.

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

Refrigerate assembled macarons for at least 24 hours before serving.