Butter the base and sides of the bowl of a slow cooker. Beat the butter and both sugars together in a stand mixer with the whisk attachment or using an electric whisk until pale and fluffy. Add the eggs, one at a time, then fold in the flour, cocoa, baking powder, espresso, a pinch of salt and the chocolate. Add just enough milk so the batter falls off the spatula in big dollops. Spoon into the prepared slow cooker.

Mix the sauce ingredients together in a heatproof bowl with 300ml boiling water, then carefully pour this over the batter in the slow cooker. Cover and cook on low for 5-6 hrs or high for 3 hrs until the cake springs back when pressed, and is well-risen. Serve with crème fraîche, whipped cream or vanilla ice cream.