Preheat the oven to 180C (350F). Line a cupcake pan with 10 paper liners. Set aside.

Sift the flour and baking powder into a bowl. Set aside.

Put the butter and peanut butter into the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed until smooth. Add the eggs one at time, beating well after each addition. Add the vanilla and beat again until you have a smooth creamy mixture.

Add half the flour mixture to the butter mixture. Mix on low speed until just incorporated. Add the buttermilk and mix until just combined. Add the remaining flour mixture and mix until just incorporated. Stop the mixer, add the chocolate chips and marshmallow fluff and gently fold them into the batter with a spatula.

Divide the batter between the tins and bake for about 18 minutes, when a skewer inserted into the centre comes out clean. Leave to cool completely on a wire rack before frosting.

Put the butter into the bowl of an electric mixer fitted with the paddle attachment and beat on medium speed until smooth. Add the marshmallow fluff and vanilla and beat for a further minute.

Turn the mixer to low speed and gradually add the icing sugar. Once fully incorporated, turn the mixer back up to medium speed and beat for 2 minutes.

Spoon the buttercream into a disposable piping bag and cut a 1 cm hole in the tip. Pipe the frosting on top of the cupcakes.

Put the extra peanut butter into a small snap lock bag. Seal the bag and put it in a cup or bowl. Pour hot water over the bag and leave for a minute to warm and soften the peanut butter to a drizzling consistency. Snip a tiny hole in one of the corners of the bag and drizzle the peanut butter over the buttercream.