Line one baking tray with a silicone sheet and another with baking parchment.
Melt sugar in a frying pan over medium heat until golden brown.
Add peanuts to the caramel and pour the mixture onto the silicone sheet to set.
Mix flour, cocoa, baking powder, soda, salt, butter, sugar, egg, vanilla, buttermilk, and coffee until combined.
Pipe 4cm circles onto the parchment-lined tray, leaving 3cm gaps between them.
Bake for 10 minutes and cool on a wire rack.
Whisk double cream with icing sugar and fold in the chopped marshmallows.
Blend smooth peanut butter, ice cream, and milk together until smooth.
Sandwich two whoopie pies together with a tablespoon of the marshmallow cream.
Serve the pies with broken brittle pieces and a shot of milkshake.