Preheat oven to 170 C / Gas 3. Butter and baseline shallow 20cm square tin with baking parchment.
Place the chocolate and butter in a bowl over a pan of simmering water. Do not let the water touch the bottom of the bowl. Heat gently until the butter and chocolate are melted. Remove the bowl from the pan.
Add the sugar to the chocolate mixture and beat. Whisk in the eggs one at a time, until combined.
Add sifted flour and beat until smooth. Fold in the fudge chunks and chocolate chips. Pour into the prepared tin and bake for 45 minutes or until the surface has set but still feels squidgy underneath.
Leave to cool completely and cut into bite sized pieces.