Break 185g of dark chocolate into small pieces in a medium sized bowl. Add 185g of unsalted butter cut into small cubes.
Fill about ¼ of a medium sized saucepan with hot water. Rest the bowl on the saucepan without it touching the water and melt the butter and chocolate over a low heat. Stir occasionally.
Once the chocolate and butter have completely melted, remove the bowl safely from the pan, cover and leave to cool to room temperature.
Heat your oven to 180C / 160C fan / gas 4.
Take a 20cm square tin and baking parchment. Cut the baking parchment to the size of the base of your tin and line.
In another medium sized bowl, sieve 85g of plain flour and 40g of cocoa powder together into the bowl.
Chop 50g milk chocolate into chunks on a board.
In a large bowl break three eggs and add 275g of caster sugar. Whisk the mixture with an electric mixer until it becomes thick, creamy, pale and doubles in volume. This can take 3-8 minutes.
Add the cooled chocolate mixture to the egg mixture and fold together gently in a figure of eight with a spatula. The mixture will start to turn to a mottled dark brown. Do this step gently and slowly to avoid losing too much air.
Grab a sieve and over your eggy, chocolate mixture, re-sieve the cocoa and flour mixture, covering the top.
Again, gently fold this mixture using the same method as before. Do this until the mixture looks gooey and fudgy. You don’t want to overdo your folding on this occasion, stop folding just before you think that you should.
Stir in 50g of milk chocolate chunks and 3tbsp of Duerr’s marmalade.
Pour your delicious, orangey, gooey mixture into the baking tin, level with a spatula and make sure it’s evenly spread across the tin.
Pop the mixture into the oven for 25 minutes. After this time, pull the oven shelf out slightly and gently shake the tin. If the brownie wobbles in the centre, it’s not quite there yet.
Bake the brownie for a little longer, until it starts to come away from the sides of the tin and has a thin, chocolatey crust across the surface. Once ready, remove from the oven and leave to cool.
Lift your brownie out of the tin and cut into even squares.