If you like your pudding gooey, chewy and chocolatey then this is the recipe for you!
Heat the oven to 180C/fan 160C/Gas 4.
Line a 22cm square brownie tin with baking parchment.
Melt the butter and chocolate together in a microwave or in a bowl set over a pan of simmering water. Leave to cool to room temperature.
Whisk the eggs and sugar together until the mixture is light and fluffy. Fold the chocolate mixture into the egg mixture and sift in the flour, baking powder and cocoa. Fold this in to give a fudgy batter.
Add 100g chopped white and milk chocolate chunks and marshmallows. Bake for 25-30 minutes or until the top is cracked but the middle just set.
Cool completely, then lift out of the tin to cut and serve.