Preheat the oven to 180°C / 160°C fan / Gas Mark 4.
Grease and line two 21cm square tins with butter and baking parchment paper.
Prepare the Blondie Mixture
Place the butter and 100g of the white chocolate in a bowl over a pan of hot water on a gentle heat and stir occasionally until the butter and chocolate have melted. Once melted leave the mixture to cool.
Combine the flour, baking powder and salt in a bowl.
Whisk the eggs and sugar together in another bowl until thick and a mousse like texture.
Add the vanilla to the chocolate mixture before pouring the mixture into the egg mixture, carefully fold together. Sieve in the flour mixture and fold again, add in the remaining 100g of white chocolate chips and stir together.
Set the mixture to one side.
Prepare the Brownie Mixture
Place the cubed Dragon butter into a medium bowl, then break up the dark chocolate and add to the butter.
Place the bowl over a small saucepan of hot water, only about a quarter full making sure the bowl isn’t touching the water. Put over a low heat until and chocolate have meted, stirring occasionally. Alternatively, you could place the bowl of chocolate and Dragon butter in the microwave, covered with cling film for 2 minutes on a high setting.
Leave the chocolate mixture to cool.
In a separate bowl sieve, the flour and cocoa powder.
In another large bowl using an electric whisk or stand mixer whisk together the eggs and the caster sugar, until thick and mousse like consistency and light in colour and doubled its original volume, this can take from 3-8 minutes. Pour in the cooled chocolate mixture, fold in the mixture trying not to lose any of the air.
Re-sieve the flour and cocoa powder into the egg mixture. Gently fold in again using a figure of eight method, try not over mix.
Stir in the chopped milk and white chocolate.
Assemble and Bake
Pour equal amounts of both mixtures into the prepared tins swirling around to create a marbled effect.
Bake in the centre of the oven for 25 minutes. The brownie should have a shiny crust and should start to come away from the sides. If the brownie wobbles live in in for a further 5 mins.
Leave the brownie in the tin until completely cold. Cut the brownie into squares or use shaped cutters.