Preheat the oven to 180c. Beat the butter, sugar, vanilla and eggs together in a large mixing bowl until smooth. Sift in the flour, cocoa powder, baking powder and salt, then combine the two mixtures to give a smooth batter. Thoroughly grease the moulds of a 12-hole muffin tin and spoon a little of the batter into each, enough to fill them halfway. Bake for 18 minutes, or until set around the edges and risen - but still slightly underdone.
Meanwhile, grease the underside of a second muffin tin with butter or oil, around the bottom and sides of the outside of each mould. As soon as you take the brownies from the oven, press the other muffin tin firmly down on to the brownies, flattening the centres of the cakes and causing their edges to splay up. After holding the tin in place for a few seconds, slowly remove it to reveal the bowl-shaped brownies.
Leave to cool completely before removing from the tin. Serve with ice-cream, sauce and sprinkles.