Line two baking sheets with silicone mats or parchment. Sift almond flour and powdered sugar together into a large bowl. Set aside.
Set a pan of simmering water on the stove. In a clean bowl, combine egg whites, golden sugar, and egg white powder (if using). Place bowl over simmering water and whisk for 2–3 minutes until sugar dissolves and mixture is warm and glossy.
Remove from heat and beat with mixer to stiff, glossy peaks (about 3–5 minutes).
Fold dry sifted almond mixture into the stiff meringue, adding red gel food coloring. Gently fold until batter flows slowly and forms thick ribbons—do not overmix (aim for 40–50 folds).
Transfer to a piping bag. Pipe 1.5-inch circles onto prepared trays. Firmly tap trays 2–3 times to remove air bubbles. Pop remaining bubbles with a toothpick if necessary.
Let shells rest at room temperature 30–60 minutes, until dry to the touch.
Bake in preheated oven at 300°F (150°C) for 15–20 minutes until firm with classic "feet." Cool completely on tray before removing.
Meanwhile, make royal icing: Whisk powdered sugar, meringue powder, and 2–3 tbsp water until smooth and thick. Divide and tint with black and yellow gel for Santa’s belt & buckle. Transfer to piping bags.
Decorate HALF of the cooled macaron shells with royal icing belt and buckle. Pipe white icing for fur trim if desired. Let dry fully.
For buttercream: Spread flour on a baking sheet and bake at 350°F (175°C) for 5 minutes to heat treat. Cool. Beat butter and sugars until fluffy, then mix in cooled flour, vanilla, almond extract, and just enough cream to make a pipeable filling.
Pipe buttercream onto undecorated shells. Sandwich with decorated tops. Roll edges in shredded coconut for “Santa’s beard.”
Chill assembled macarons in an airtight container at least 24 hours before serving for best texture (if you can wait that long!).