In a food processor, add the cold butter, sugar, salt, and vanilla extract. Mix at low speed for a few seconds until the butter is all chopped up.
Add the egg, flour, and baking powder and mix again for a minute or two until the dough starts coming together.
Divide the mix into 2 bowls and in one add the cocoa powder. Knead well with your hands until the dough comes together into a smooth and solid ball. At this point, it shouldn't crumble anymore.
Cover both doughs in plastic wrap and flatten it into a disk so it will be easier to roll out.
Chill in the fridge for at least 30 minutes.
Preheat the oven at 350°F / 180°C.
Take the dough out of the fridge and place it on a wooden board or table dusted with flour. Roll it out with a rolling pin about ¼ inch (6mm) thick and cut out the shapes with cookie cutters.
Transfer the cookies to a lined cookie sheet and bake for 10 minutes.
Take them out of the oven and transfer them immediately (carefully!) to a cooling rack and let them cool down completely before you decorate them.
You can melt a couple of tablespoons of dark chocolate for a minute in the microwave and drizzle the melted chocolate on the cookies with a teaspoon.
Notes
You can store the cooled cookies in an airtight container or tin cookie box for up to 2 weeks. You can also freeze the cooled cookies for up to 3 months.