Step 1

Preheat oven to 180°C. Grease and line the base and sides of an 18cm square cake pan.

Step 2

Place chocolate in a heatproof bowl over a saucepan half-filled with simmering water. Stir with a metal spoon for 5 minutes or until melted. Set aside for 5 minutes to cool slightly.

Step 3

Use an electric mixer to beat butter and sugar in a large bowl until pale and creamy. Add eggs, one at a time, beating well after each addition. Add chocolate and beat.

Step 4

Add flour, cocoa powder, baking powder and bicarbonate of soda and stir until just combined. Add buttermilk and stir to combine. Spoon into the prepared pan and smooth the surface. Bake for 45 minutes or until a skewer inserted in the centre comes out clean. Cool in pan for 5 minutes before turning onto a wire rack to cool completely.

Step 5

To make frosting, place chocolate and butter in a heatproof bowl over a saucepan half-filled with simmering water. Stir with a metal spoon for 5 minutes or until melted and smooth. Remove from heat. Add icing sugar and stir well. Set aside for 30 minutes or until icing is firm, then use a wooden spoon to beat until smooth. Spread over the top of the cake. Serve or store in an airtight container for up to two days.