Step 1

Preheat the oven to 160C/140C fan forced. Grease the base and side of a round 22cm cake pan with melted butter. Line the base with baking paper.

Step 2

Melt the chocolate in a heatproof bowl over a saucepan of simmering water, stirring occasionally, until smooth (make sure the bowl doesn’t touch the water). Remove from heat. Set aside to cool slightly.

Step 3

Use electric beaters to beat the butter and sugar in a large bowl until pale and creamy. Beat in the melted chocolate, then beat in the eggs and vanilla until well combined. Beat in the cocoa powder, bicarb, sour cream and boiling water until combined. Add the flour and stir until smooth and combined. Pour into the prepared pan and smooth the surface. Bake for 1 hour 15 minutes or until a skewer inserted into the centre comes out clean. Set the cake aside to cool in the pan.

Step 4

Meanwhile, to make the frosting, use electric beaters to beat the cream cheese and butter in a bowl until smooth and creamy. Gradually beat in the icing sugar, cocoa and milk until smooth.

Step 5

Turn the cake out onto a serving plate. Spread the frosting over the top, using the back of a tablespoon to create swirls.