Preheat the oven to 180c/gas mark 4/350f
In your mixing bowl, mix together the flour, cocoa powder, brown sugar, baking soda and salt.
In a separate mixing bowl, whisk up the stout beer, the oil, vinegar and the vanilla.
Pour the wet mixture into the dry mixture and mix until both mixtures are fully combined. The paste should be relatively thick so don't over mix by whisking too much.
Line 2 cupcake/muffin trays with 14 cupcake cases and spread out the mixture evenly between these cases, filling about 2/3 of the cupcake case.
Bake between 15-20 minutes or until a toothpick inserted in the centre of the cupcake comes out clean with a few cooked crumbs on the stick.
When ready, remove the cupcake and leave to cool on a wire rack before decorating.
In a sauce pan, melt the butter and sugar together over a low heat until the sugar is dissolved but be careful not to burn.
Slowly stir in the cream or milk and heat gently until the mixture is bubbling, be sure to stir continuously.
Remove from the heat allow to slightly cool.