Preheat the oven to 180°C.
Place a saucepan with 2.5cm of boiling water over heat to simmer.
Grease a 23cm square tin and line it with baking parchment.
Chop the dark chocolate into small pieces and place in a bowl with the butter.
Melt the chocolate and butter over the simmering water, stirring occasionally, then remove from heat.
Chop the white or milk chocolate into penny-sized pieces.
Whisk the sugars and eggs into the melted chocolate until bubbly.
Sift the flour, cocoa, and salt, then fold into the wet mixture in three batches.
Pour batter into the tin, top with chocolate chunks, and bake for 20-25 minutes.
Cool completely in the tin before slicing into squares.