In a bowl, combine the flour, cocoa powder, salt, and baking powder.

In another bowl, add the sugar, butter, egg, vanilla, and milk. Whisk well to combine.

Add the dry mixture to the wet one and mix with a rubber spatula just until no dry ingredients remain.

Wrap the dough in plastic wrap and freeze for 1 hour or until it's solid enough to form into dough balls. You can also refrigerate the dough overnight.

Preheat the oven to 175°C (350°F) and line a baking sheet with parchment paper.

Form 12 cookie dough balls (about 30g each) and roll them in sifted powdered sugar. Be sure to cover the dough balls generously with sugar.

Place the cookie balls on the baking sheet with at least 2 inches (about 4 cm) of space between each and bake for about 12-15 minutes or until they have spread and are completely set.

Remove from the oven and let them cool for 5 minutes on the sheet before transferring them onto a cooling rack to cool completely.