Step 1

Preheat oven to 180°C. Brush a 20 x 30cm (base measurement) shallow cake pan with melted butter to lightly grease. Line the base and 2 long sides with non-stick baking paper, allowing the sides to overhang.

Step 2

Place the chocolate and butter in a heatproof bowl over a saucepan half-filled with simmering water (make sure the bowl doesn't touch the water). Stir with a metal spoon until the chocolate melts and the mixture is smooth. Set aside to cool slightly.

Step 3

Combine the egg, sugar and vanilla in a large bowl. Stir in the chocolate mixture. Add the flour, baking powder, walnut and extra chocolate, and stir until well combined. Pour into the prepared pan. Bake for 40-45 minutes or until a skewer inserted into the centre comes out clean. Set aside in the pan for 10 minutes to cool before turning onto a wire rack to cool completely.

Step 4

To make the chocolate frosting, use an electric beater to beat the butter and icing sugar until pale and creamy. Add the chocolate and beat until fluffy. Spread over the brownie. Trim the edges and cut into squares to serve.