Preheat the oven to 180 C / Gas mark 4. Grease and line a 20cm square baking tin with greaseproof paper

In a pan over low heat, melt the dark, milk and white chocolates and butter, stirring constantly until melted.

Whisk the eggs and sugars together in a large bowl until thick and light in colour. Stir in the melted chocolate and vanilla essence. Gently fold in the flour and baking powder, mixing until just combined; pour the mixture into the prepared baking tin.

Bake in the preheated oven until the brownie has set up but is still moist in the centre, 30 to 40 minutes.