Cream the softened butter on medium speed until light and fluffy. Gradually add powdered sugar, mixing until airy. Add egg and beat until just combined.

In a separate bowl, whisk together flour, black cocoa powder, Dutch cocoa powder, and salt. Gradually add dry mixture to wet batter on low speed until combined.

Turn dough onto floured surface, shape into a disc, wrap in cling film, and chill in refrigerator for at least 30-45 minutes.

Roll chilled dough between two parchment papers to 4mm thickness.

Cut out shapes with cookie cutters, transfer to a lined baking tray, cover, and chill for an additional hour.

Preheat oven to 160°C (320°F). Bake cookies for about 10 minutes until edges feel firm but centers remain soft. Cool completely before decorating.

Beat egg whites until foamy. Gradually add powdered sugar, mixing until peaks form. Add gel food coloring if desired.

Using piping bags, decorate cooled cookies and allow icing to dry for 3-6 hours.