Preheat the oven to 180°C.
Grease a 20cm square baking tin with butter and line it with baking paper.
Melt the butter in a pan over medium heat, then stir in the chocolate until smooth.
Whisk eggs, egg yolks, and vanilla until light and fluffy.
Add sugar in two stages, whisking until the mixture becomes stiff.
Fold the melted chocolate mixture gently into the eggs.
Stir in the flour, cocoa powder, salt, and one-third of the biscuit pieces.
Pour mixture into the tin and scatter remaining biscuits on top.
Bake on the middle shelf for 25–30 minutes until slightly gooey in the center.
Cool in the tin, then remove and cut into squares before dusting with icing sugar.