In a large bowl, sift together the confectioners sugar and cocoa, and set aside.
In another large bowl, cream butter until smooth.
Gradually add in one quarter of the sugar mixture, 2 Tbsps at a time.
Add in all of the vanilla, then another quarter of the sugar mixture, two tablespoons at a time.
The mixture should be firm. At this point, gradually add the milk, about a 1/2 tablespoon at at a time, alternating with 2 tablespoons at a time of the rest of the sugar mixture.
Classic Vanilla Cupcakes.
Preheat oven to 350 degrees.
Put the flour, sugar, baking powder, salt and butter in an electric mixer with a paddle attachment (or use a hand-held electric whisk) and beat on slow until you get a sandy consistency and everything is combined.
Gradually pour in half the milk and beat until the milk is just incorporated.
Whisk the eggs, vanilla extract, and remaining milk together in a separate bowl for a few seconds, then pour into the flour mixture and continue beating until just incorporated (scrape any unmixed ingredients from the side of the bowl with a rubber spatula).
Continue mixing for a couple more minutes until the mixture is smooth. Do not over-mix.
Pour the mixture into a muffin pan lined with paper cases until two thirds full and bake in the preheated oven for 20–25 minutes, or until light golden and the sponge bounces back when touched (smaller cupcakes will need 3-5 minutes less baking time).
A toothpick inserted in the center should come out clean.
Leave the cupcakes to cool slightly in the tray before turning out on to a wire cooling rack to cool completely.
When the cupcakes are cold, frost with chocolate buttercream frosting.