For the chocolate and ale cake: Preheat the oven to 350 degrees F. Grease and flour a 10-inch loaf tin.
Slowly mix the dark ale into the cocoa powder in a bowl to form a paste. In a separate bowl, beat the butter and dark brown sugar together until smooth and creamy, then add the eggs one at a time, beating between each addition. Stir in the cocoa and ale mixture. Sift in the flour, baking soda and baking powder in batches, stirring between each addition.
Pour the cake batter into the prepared tin and bake until springy to the touch, 30 to 35 minutes. Cool on a wire rack.
For the ale and chocolate icing: Melt the chocolate in a heatproof bowl suspended over a pan of simmering water. Beat the confectioners' sugar and butter together in a bowl to form a soft paste. Add the dark ale and then fold in the melted chocolate.
For the muscovado and malt cream: Whip together the cream, malt, muscovado sugar and salt in a large bowl until thick and smooth.
When the cake has cooled completely, cut it into slices. Serve with the ale and chocolate icing spread over the cake and a dollop of the muscovado and malt cream on the side.
Double cream, which is similar to heavy cream and has a butterfat content of 48 percent, can be found in specialty stores.