In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 1-2 minutes.

Add the egg, vanilla extract, salt, baking powder, and cocoa powder. Mix until combined, then add the all-purpose flour and mix until smooth.

Lay a sheet of wax or parchment paper on a work surface. Place half of the cookie dough in the center and press it into a disk. Cover with another sheet of paper and roll to between 1/4 to 1/2 inch thickness, as desired. Repeat with the second half of the dough.

Chill the dough until firm, at least 30 minutes.

Preheat oven to 350°F (175°C). Line cookie sheets with parchment paper or silicone baking mats.

Remove half of the dough from the refrigerator and use cookie cutters to cut desired shapes. Place the cut-out cookies about 2 inches apart on the prepared cookie sheets. Chill for another 15 minutes before baking. Bake cookies until they no longer look glossy, approximately 10-15 minutes, depending on thickness and size.

Reroll any leftover dough and chill while you cut shapes from the second half. Continue until all the cookie dough has been used.

Once the cookies are baked, let them cool completely on the baking sheets before transferring them to a wire rack to cool further.

To make the frosting: Beat the softened butter and powdered sugar with a stand mixer until crumbly. Add the salt and vanilla extract. Gradually add the milk, 1 tablespoon at a time, mixing until smooth. Add more milk, a teaspoon at a time, if needed to reach a piping consistency.

If desired, divide the frosting and color it with food coloring. Frost the cooled cookies as desired. Let the frosting set at room temperature until it hardens slightly, then you can stack the cookies.