Preheat the oven to Gas Mark 4, 180°C, fan 160°C. Line a 23cm (9in) square tin with nonstick baking paper. Melt the milk chocolate in a bowl set over a pan of simmering water. Remove from the heat. In a separate bowl, melt the white chocolate in the same way. Using an electric whisk, beat the butter and sugar. Add 3 eggs, one at a time, beating after each. Whisk for 3-4 mins. Using a large spoon, fold the melted milk chocolate into the butter mixture. Fold in the flour, baking powder, cocoa powder and salt. Add the cream cheese and remaining egg to the melted white chocolate. Whisk on a low speed until smooth. Pour half the milk chocolate mix into the tin; spoon over alternating layers of each mixture. Swirl a knife or skewer through the mix to create a marbled effect. Transfer to the oven and bake for 45 minutes until almost set (the centre should have a slight wobble). Leave to cool for at least 1 hour before slicing into brownies.