Preheat oven to 180 degrees celcius (gas mark 4). Grease and line the base and sides of a 20cm square cake tin with baking parchment.
Melt the curd and butter in a bowl over a pan of simmering water. Cool slightly.
Whisk the eggs and sugar until thick and creamy, pour over the chocolate mixture and fold in.
Sift in the flour, cocoa and fold in the nuts (if desired). Pour into the prepared tin, making sure it goes right into the corners and bake for 25-30 minutes. The top should be crusty with a slight wobble underneath.
Cool completely in the tin (can refrigerate overnight which aids cutting) and then cut into squares or triangles.