Pre-heat oven to 180°C. Line a 20cm cake tin with baking paper.
Combine butter and chocolate in a heatproof bowl over a saucepan of simmering water, making sure the bowl is not touching the water. Stir until the butter and chocolate have melted. Remove from heat and cool to lukewarm.
Using an electric mixer, beat the sugar, eggs and zest until pale and creamy. Fold through the flour to mix in. Then fold through the chocolate mixture, then the nuts. Pour into prepared tin. Bake for 25 minutes. Skewer should have moist crumbs when inserted. Cool in tin.
Cut cake into 5cm squares.