Prepare all the ingredients. Cream the butter with the sugar. In a separate bowl, mix the flour with the cocoa powder, baking soda, salt, and baking powder.

Once the butter is creamed, add the eggs, vanilla, and milk. Beat well until fully combined. Chop the walnuts in a food processor until finely chopped, with some small pieces remaining.

Gradually add the flour mixture to the mixer, beating well after each addition. If the dough becomes too thick, finish mixing by hand.

Add the walnuts and mix until just combined. Shape the dough into a ball and refrigerate for at least one hour so it firms up. Preheat the oven to 350°F (180°C).

Shape the dough into marble-sized balls, about 1/2 inch (1 cm) in diameter, and place them on a baking sheet. Bake for 10 to 12 minutes, until they look set and slightly cracked on top. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack and cool until firm. Place the powdered sugar in a bowl and gently roll the cookies to coat. I only roll them once so they're not too sweet, but you can roll them twice if you like. Enjoy!