Melt butter and mix in cocoa until it’s thoroughly blended. Add brown sugar. Let it cool slightly, then mix in beaten eggs. Add baking powder, salt, and vanilla and stir. Add flour and mix thoroughly.
Chill dough in the refrigerator for at least 1 hour or overnight.
Preheat the oven to 180 C / Gas 4.
Roll dough into small balls with your hands. This dough may be sticky, so roll only enough for the cookies you plan to bake immediately, then return the bowl to the refrigerator.
Roll the dough balls in the caster sugar, flatten them slightly and place them on greased baking sheets, 12 to a standard sheet.
Bake for 10 minutes. Cool on baking sheets for a minute or two and then immediately remove the cookies to a wire rack to finish cooling. If you leave them on the baking sheet for too long, they’ll stick!