First, preheat your oven to 180°C (160°C fan/Gas Mark 4). Grease and line a 20cm (8-inch) square baking tin with parchment paper, leaving some overhang on the sides to make it easier to lift out later.

To make the brownie batter, place the butter and dark chocolate in a heatproof bowl set over a saucepan of gently simmering water (a bain-marie). Make sure the bottom of the bowl doesn't touch the water. Stir occasionally until completely melted and smooth, then set aside to cool slightly.

In a separate large bowl, using an electric mixer or a stand mixer with the whisk attachment, beat the eggs, caster sugar, and light brown sugar together for about 3-5 minutes. The mixture should be pale, thick, and have significantly increased in volume.

Pour the cooled chocolate and butter mixture into the egg mixture, add the vanilla extract, and gently fold everything together with a spatula until just combined.

Sift the plain flour, cocoa powder, and salt over the wet ingredients. Fold them in gently until you can no longer see any streaks of flour. Be careful not to over-mix at this stage. What works best for me is using a slow figure-of-eight motion.

Now for the cheesecake layer. In a medium bowl, beat the room temperature cream cheese, caster sugar, egg yolk, and vanilla extract with an electric mixer until it's completely smooth and creamy. This should only take a minute or two.

Pour about two-thirds of the brownie batter into your prepared tin and spread it out evenly. Dollop the cheesecake mixture on top in large spoonfuls. Then, add the remaining brownie batter in smaller dollops in the gaps.

Take a skewer or the end of a teaspoon and gently drag it through the batters to create a marbled, swirled pattern. Don’t overdo it, or the colours will become muddy.

Finally, press the fresh blackberries gently into the top of the batter. Some will sink slightly, which is exactly what you want.

Bake in the preheated oven for 35-40 minutes. The edges should be set and slightly puffed, but the centre should still have a very slight wobble when you gently shake the tin. This is the key to a fudgy centre.

Leave the brownies to cool completely in the tin on a wire rack. Once cool, you can place them in the fridge for a couple of hours to firm up, which makes slicing much neater.