- Preheat the oven to 200⁰C, gas mark 6. Grease and line the Swiss roll tin with baking parchment.
- Using an electric hand mixer, whisk the eggs and sugar together in a large bowl until thick and creamy and leaves a trail. Sieve the flour and cocoa together and fold into the mixture carefully with 1 tbsp hot water, trying not to knock out too much air.
- Place into the prepared tin and tilt the tin to level the mixture, making sure it goes right into the corners. Bake for 10-12 minutes until springy to the touch.
- Dust a large piece of baking parchment with caster sugar and turn the sponge out onto it. Trim off the hard edges and make a dent along the width, 1cm in. From this end, roll it up with the paper in the middle. Cool on a wire rack.
- Beat the butter and icing sugar together until light and fluffy and then beat in the melted chocolate. Unroll the sponge, spread over the butter icing and up roll again. Serve in slices.