Preheat your oven to 350 degrees F. With an electric mixer, cream the butter and sugar for five minutes. Then, add the eggs, one at a time, and add the vanilla extract.
In a bowl, sift together the flour, the cocoa, the baking soda and the salt, and the coffee grounds. In another bowl, mix the milk, the sour cream and the brewed coffee.
Add the two mixtures to the bowl with the butter, the sugar and the eggs. It is best to alternate adding the dry and wet ingredients. Mix well.
Put the batter into muffin tins lined with cupcake papers. Bake them for 20 minutes. (Rotate after 10 minutes, check at 18 minutes.) Let cool in pans for 5 minutes, then remove to cool completely on racks.