Preheat the oven to 180°C.

Put the plain chocolate and butter in a large heatproof bowl, place over a pan of simmering water and allow melting. Stir until smooth and set aside to cool a little.

Whisk the eggs and sugar together with the vanilla seeds until it forms a nice creamy froth. Lightly fold in the melted chocolate then sift in the flour and cocoa powder and mix through gently. Stir in the chunks of milk and white chocolate.

Grease the baking tin with butter and then line with baking parchment or greaseproof paper. Carefully pour the chocolate mixture into the lined dish.

Bake in the oven for 25-30 minutes. The top should be firm and crispy but the inside should still be moist. Allow to cool on a wire rack before cutting into squares and serving at room temperature.