Line the baking tin with parchment paper then set it aside. Preheat the oven to 180C/350F.

In a small mixing bowl, melt the butter in short bursts in the microwave, stirring in between, until melted. Tip in the cocoa powder and mix to combine. Set this bowl aside to cool slightly.

In a separate large mixing bowl, beat together the sugar and eggs for 2 minutes until pale and creamy. Add in the vanilla and chocolate mixture and stir together to combine.

Sift in the flour and salt and fold it into the chocolate mixture until almost combined. Tip in the chocolate chips and fold in, making sure there are no streaks of flour remaining. Set this bowl aside whilst making the blondie mixture.

In a large mixing bowl, combine the butter, sugar and white chocolate. Melt them together in the microwave in 10-20 second bursts, stirring in between, until completely melted. Once everything has melted, keep mixing these ingredients until they come together and turn into a smooth fudgy mixture. The mixture may look curdled to start with. Just keep mixing, be patient, and have faith it will come together in the end!

Add the eggs and vanilla into the bowl one at a time, mixing well to combine in between additions. The mixture will be thick and glossy.

Sift in the flour and salt, then fold in gently to almost completely mix together. Add in the chocolate chips and fold together to mix in thoroughly, leaving no streaks of flour.

Once both brownie and blondie mixtures have been made, it is time to bring them together to bake. Using a large spoon to measure each mixture, place alternating dollops of each mixture around the base of the tin. Once the base has been covered, place spoonfuls of mixture on top, trying to keep blondies over brownies or brownies over blondies. Keep doing this until all of both mixtures have been used up.

Using a knife, gently swirl the mixture around in the tin to create a marbled pattern on top of the brownie blondie. Be careful not to swirl too much or both mixtures will combine and lose definition.

Bake in the preheated oven for 40-50 minutes until the edges are firm and shrinking away from the sides of the pan, and the middle no longer wobbles. A wooden skewer inserted into the middle of the bake will come out with a few very moist crumbs. Remove from the oven and cool on a rack for 5 minutes before turning out of the tin. Leave to cool completely before cutting into squares. Enjoy!