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Heat oven to 180C/160C fan/gas 4. Put the flour, cocoa powder, sugar, baking powder, a pinch of salt and the butter in a free-standing electric mixer with a paddle attachment (or use a handheld electric whisk). Beat on a slow speed until you get a sandy consistency and everything is combined.
Whisk the milk, egg and vanilla extract together in a jug, then slowly pour about half into the flour mixture. Beat to combine and turn the mixer up to high speed, scraping any mixture from the side of the bowl with a rubber spatula. Pour in the remaining liquid and continue mixing for a couple more minutes until the mixture is smooth. Do not overmix.
Spoon the mixture into the paper cases until approximately two-thirds full. Bake in a preheated oven for 20-25 minutes, or until the sponge bounces back when touched and a skewer inserted into the centre comes out clean.