Chocolate Muffins with Oreo Buttercream 🇺🇸

MUFFINS: 60 g cocoa powder 150 g self-raising flour 280 g light brown soft sugar 3 eggs 150 ml milk 130 ml vegetable oil OREO BUTTERCREAM: 6 Oreo cookies 175 g butter, softened 350 g icing sugar ½ teaspoon vanilla extract 1-2 tablespoons milk TO DECORATE: 6 Oreo cookies