Place butter, sugar and vanilla in a large mixing bowl and beat with a stand or electric mixer until combined. Add the eggs, one at a time, and mix on medium-low speed just until combined - about 10 seconds each.
Add cocoa powder, flour, baking powder and salt in a medium bowl and mix on low speed until incorporated.
Cover the bowl and chill the dough for at least 1 hour or up to overnight (the longer you chill, the thicker the cookies will be).
When ready to bake, preheat the oven to 350°F. Place the powdered sugar in a medium bowl. Roll the dough into tablespoon-sized balls and cover them well with the sugar.
Place on a lined baking sheet with enough space between them and bake for 10-15 minutes, or until spread and crackled. The cookies will look soft in the center but will firm up as they cool. Don't over bake.
