'Whoopie Pies' are American cakes which are very popular over here now! I couldn't find a 'non-American' recipe (I wasn't sure if one 'cup' was the same as one Hollyoaks mug) so I just guessed and use these amounts:
And here are the simple instructions:
Melt chocolate and butter. Whisk sugar, eggs and vanilla together. Sift flour, cocoa powder, baking powder and salt into a bowl. Mix everything together! Bake for about 10-15 mins. Then just butter icing for the middle.