Step 1

Heat the oven to 180C (160C fan)/350F/gas 4.

Step 2

Grease a 23cm x 33cm baking tin, then line it with greaseproof paper, leaving enough overhang to help you lift out the cake once baked.

Step 3

For the cake, put the chocolate and butter in a heatproof bowl and melt in short bursts in a microwave or over a pan of simmering water. Set aside to cool.

Step 4

In a large bowl, mix the flour, cocoa powder, sugar, baking powder, bicarb and salt.

Step 5

Make a well in the centre, pour in the eggs, oil, milk and cooled chocolate/butter mix, and whisk to a thick, smooth batter. Pour in 150ml hot water and mix again until well combined.

Step 6

Pour the batter into the prepared tin, then bake for 35-40 minutes, or until a skewer inserted into the middle comes out clean. Take out of the oven and leave to cool.

Step 7

Meanwhile, make the frosting. Put the chocolate, butter, cocoa powder, half the cream and the salt in a small saucepan and heat very gently until just melted.

Step 8

Put the mascarpone in a bowl, pour the chocolate mixture over it, then whisk well to combine.

Step 9

Add the rest of the cream, mix until smooth, then leave to cool and thicken to to a spreadable consistency.

Step 10

Spread the frosting all over the top and sides of the cake, top with shaved chocolate, then slice and serve.