Melt the butter in a large pan over a medium heat, then stir in the flour to form a paste.

Add the flour, stir to form a paste and cook for a minute.

To make the cake: mix the dark ale into the cocoa powder to form a paste.

Mix the cornflour with cold water to form a paste.

While still hot, place in a food processor and blitz until they form a paste and begin to release their oils.