Preheat the oven to 170 degrees C (338 degrees F). Line a 22-by-29-centimeter (8 1/2-by-11-inch) brownie tin with baking parchment.
For the brownie mix: Break the 225 grams chocolate into small chunks and place in a heatproof bowl with the butter. Place over a pan of simmering water and heat until melted. Leave to cool.
Beat the eggs and sugar together until pale and thickened. Add the melted chocolate-butter mixture to the eggs and stir until thoroughly combined. Fold in the flour, salt, 175 grams chocolate chunks, and vanilla extract. Pour into the prepared in.
For the cheesecake mix: Using an electric whisk on a slow speed, beat the cream cheese with the custard powder, sugar, vanilla extract and double cream. Mix until smooth and all the ingredients are combined.
Drop spoonfuls of the cheesecake mix onto the brownie mix and use a spatula to fold it through the brownie, creating swirls.
Bake for 25 to 30 minutes. Check after this time as ovens vary; it may need a little longer. When done, a little mixture should still stick to a skewer inserted in the middle of the brownie.
Leave to cool completely before cutting into squares.