Start by preheating your oven to 180ºC (160ºC fan) and line a muffin tin with 12 baking cases. Beat butter and sugar until light and fluffy, then mix in flour, cocoa, and eggs until combined. Divide evenly and bake for 20-22 minutes until fully set.
Beat the butter alone for a few minutes, then add icing sugar and cocoa powder. Mix in vanilla extract and slowly incorporate evaporated milk until you get a smooth, fudgey frosting that pipes beautifully.
Pipe the chocolate frosting onto cooled cupcakes using your favorite tip. Add two pretzel halves for antlers, a red candy for the nose, and the edible eyes to complete your festive reindeer look.