1. Prepare the Crust:

1. Sift together almond flour and powdered sugar. In a separate bowl, whip egg whites to soft peaks, then gradually add granulated sugar until stiff peaks form. Gently fold the dry ingredients into the meringue in three additions until the batter flows like lava. Divide and color the batter, then pipe into 1.5-inch circles on a parchment-lined baking sheet. Tap the sheet firmly to release air bubbles and let rest for 30-60 minutes until a skin forms.
2. Preheat oven to 300°F (150°C). Bake the macarons for 15-18 minutes, rotating the tray halfway through. Let cool completely on the baking sheet before carefully removing them.
3. For the buttercream, beat the softened butter until light and fluffy. Gradually add the powdered sugar, then the vanilla and enough cream to reach a pipeable consistency. Pair the macaron shells by size and pipe a dollop of buttercream onto the flat side of one shell, then sandwich with its pair.
4. For the royal icing, whisk the egg white and lemon juice until frothy. Gradually add the powdered sugar and whisk until smooth and thick. Transfer to piping bags with small tips and decorate the macaron tops with festive Christmas designs like trees, snowflakes, and ornaments. Let the icing set completely before serving.