In a medium bowl, sift together the flour, cocoa, baking powder and salt. Then whisk until evenly combined.
In a separate large bowl, beat together the butter, brown sugar and granulated sugar until combined and you no longer see pieces of butter.
Mix the egg and vanilla into the butter mixture.
With the mixer on a low speed, carefully mix the dry ingredients into the butter mixture. The dough should be thick.
If the dough feels sticky, cover with plastic and refrigerate for 30 minutes so that it's easier to work with.
Scoop the dough into balls with about 1 tablespoon of dough each. Roll into balls. Using your thumb or the back of a small measuring teaspoon, press an indent into the middle of each cookie.
Cover the cookie dough balls and freeze for at least 2 hours. Optionally, frozen cookie dough balls can be placed in a freezer bag (once they are firm enough to hold their shape) and frozen for up to 2 months.
When ready to bake, preheat the oven to 350F (180C) and line cookie sheets with parchment paper or baking mats. Place the cookies about 1.5 inches (3-4 cm) apart on the lined cookie sheets. Do not thaw the cookies before baking.
Bake 1 sheet at a time in the middle of the oven for about 11-14 minutes or until the tops look just set.
Take the cookies out of the and cool on the cookie sheets. If they have started to lose the indent, very gently press the back of a measuring teaspoon into the middle of the cookies. This must be done when they are fresh out of the oven.