In a stand mixer, beat the room temperature butter on medium speed for about 2 minutes until creamy.

Gradually blend in the powdered sugar, mixing for another 3–4 minutes until light and fluffy.

Add the room temperature egg and mix on low speed until fully incorporated.

In a separate bowl, sift together the all-purpose flour, black cocoa powder, Dutch process cocoa powder, and salt.

Gradually add the dry mixture to the wet ingredients in the stand mixer and mix on low speed until just combined.

Form the cookie dough into a ball, flatten into a disc, wrap in plastic wrap, and chill in the refrigerator for 30–45 minutes.

Roll the chilled dough between two sheets of parchment paper to a thickness of about 4 mm and use cookie cutters to shape.

Transfer cut-out shapes to a baking sheet lined with parchment paper and chill for 1 hour.

Preheat the oven to 320°F (160°C) and bake the cookies for about 10 minutes.

Allow cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

Prepare royal icing and decorate cooled cookies, letting icing dry for 3–6 hours.