Sift the dry ingredients. Whisk 1 1/2 cups flour, 1/4 cup cocoa powder, the pinch of salt, and the baking powder, if using, then sift to remove cocoa lumps. Cream the butter. Beat 1/2 cup softened butter with 2/3 cup powdered sugar for 2 to 3 minutes, until smooth and creamy.
Add the egg. Mix in the egg and 1 teaspoon vanilla until the batter looks uniform, scraping the bowl once. Form the dough. Add the dry mixture on low speed and stop as soon as a soft, sticky dough forms.
Chill the dough. Flatten the dough into a disk, wrap it, and refrigerate for 20 minutes until firm. Roll and freeze. Roll between two sheets of parchment to 1/8 inch thick, then freeze the sheet on a tray for 10 to 15 minutes.
Cut the cookies. Cut shapes, place them 1 inch apart on parchment lined sheets, and reroll scraps once or twice, chilling again if soft.
Bake. Bake at 350 degrees F for 8 to 10 minutes, until the edges are firm and the centers are slightly soft.
Cool. Rest on the sheet for a few minutes, then cool completely on a wire rack before decorating.