Sift the powdered sugar and almond flour together using a whisk to eliminate clumps and ensure a fine mixture. Set aside and line sheet pans with silicone mats.

Whisk the egg whites at medium speed until small bubbles appear. Gradually add 3 tablespoons of granulated sugar, one tablespoon at a time, continuing until stiff peaks form. Add vanilla extract and brown gel food coloring, whisking until glossy and stiff.

Gently fold the powdered sugar and almond flour mixture into the meringue in three additions. Stop folding when the batter is smooth and flows slowly off the spatula.

Pipe ¾ inch circles onto the lined sheet pans. Tap the tray on the counter to release air bubbles. Let rest until a skin forms, about 20 minutes, then preheat the oven to 315°F (160°C).

Bake the macarons for 14 to 16 minutes at 315°F. Remove from the oven and let cool completely on the silicone mats.

Combine powdered sugar, meringue powder, and water in a bowl to achieve a smooth icing texture. Divide the icing and tint each portion with different gel food coloring.

Mark where the royal icing will go for the decorations on the macaron shells. Pipe the colored icing as needed and allow to dry thoroughly, about one hour.

Whisk the butter until smooth, gradually adding powdered sugar. Add heavy cream and cocoa powder, mixing until creamy.

Fill an 8 inch pastry bag with the chocolate buttercream and pipe a dollop onto each bottom macaron shell. Sandwich with the decorated top shells. Refrigerate assembled macarons for at least 24 hours before serving.