Preheat the oven to 180C, 350F/gas mark 4. Line a 1kg loaf tin with greaseproof paper. Put the butter and Guinness in a saucepan over a low heat until the butter has melted.
Sift the flour, cocoa powder and bicarbonate of soda together in a mixing bowl. Add the sugar, milk, egg, vanilla, chocolate pieces and melted butter mixture and mix together well.
Spoon the mixture into the prepared loaf tin and bake in the preheated oven for 50 minutes until risen and just firm to touch. A skewer or cocktail stick inserted into the middle of the cake should come out clean.
Leave to cool for 10 minutes, then carefully remove from the tin. It is delicious served warm as a dessert with a chocolate sauce, or eaten cold.
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