Cream the softened butter in a stand mixer until creamy, then gradually add powdered sugar and cream until light and fluffy. Incorporate the room temperature egg.

Whisk together flour, black cocoa powder, Dutch cocoa powder, and salt in a separate bowl. Gradually mix dry ingredients into the butter mixture.

Shape the dough into a disc, wrap in cling film, and chill in the refrigerator for at least 30 minutes, up to 2 days.

Roll out the chilled dough between two sheets of parchment paper to about 4 mm thick. Use cookie cutters to cut out shapes.

Chill the cut cookie shapes on a lined baking tray for 1 hour.

Preheat oven to 160°C (320°F) and bake cookies for about 10 minutes. Cool on a wire rack.

Beat egg whites until frothy, gradually add sifted powdered sugar, and mix until glossy peaks form. If desired, color using gel food coloring.

Decorate cooled cookies using piping bags to create fun designs and let sit for 3 to 6 hours until icing dries completely.