Sift together powdered sugar and almond flour into a large bowl. Repeat the sifting twice.

In a mixing bowl, whip egg whites on medium speed until foamy. Add cream of tartar, then gradually add granulated sugar until stiff peaks form.

Fold in green gel food coloring until evenly mixed. Then, gently add almond flour and powdered sugar mixture, combining until smooth.

Transfer batter to a piping bag. Pipe tree shapes on parchment-lined baking sheets, letting them sit for 30-60 minutes.

Preheat the oven to 300°F (150°C). Bake macarons for 14-16 minutes, rotating trays halfway through.

Beat softened butter until light and fluffy. Gradually mix in powdered sugar, then add heavy cream, vanilla extract, and salt.

Turn half of the cooled macaron shells upside down. Pipe filling onto each shell and sandwich with another shell.

Refrigerate assembled macarons in an airtight container for at least 24 hours before serving.