Line a muffin tray with cupcake cases.

Put the butter and chopped dark chocolate in a large, heatproof bowl. Gently melt until smooth. You can use the microwave to do this, using 30 second bursts and stirring after each burst. Alternatively, you can put an inch of water in a pan, place the bowl on top (so that the water isn't touching the bowl) and bring to the boil so that the steam gently melts the chocolate, stirring as you go.

Whisk the sugar in. The mixture will thicken - graininess at this stage is fine.

If the mixture is hot, wait until it's just slightly warm before adding the beaten egg and vanilla. Whisk together until smooth.

Sift the flour into the bowl. Gently fold through until evenly combined.

Fold in the chocolate chips.

Share the mixture evenly between the cupcake cases.

Place on a wire rack to cool for 15 minutes.

If you wish them to be firmer, place the in the fridge to chill overnight.

Put the milk chocolate and dark chocolate in a heatproof bowl and melt in the microwave in 15-20 second bursts until just melted.

Whilst whisking the chocolate, gradually pour in the cream, whisking until thick and glossy.

Leave to cool for 30 minutes or until thick enough to pipe.

Transfer the frosting to a piping bag fitted with a closed star nozzle and pipe swirls on top of each brownie cupcake. Allow to set for 30 minutes before serving.