Prepare Your Tin: Preheat your oven to 170°C (150°C fan/Gas Mark 3). Line a 23cm x 33cm (9in x 13in) baking tin with baking parchment, leaving an overhang on two sides to help lift the brownies out later. The paper should sit smoothly against the sides.

Melt the Chocolates and Butter: In a large, heavy-bottomed saucepan over a low heat, melt the cubed unsalted butter and both the dark and milk chocolates, stirring gently until the mixture is smooth and glossy, with no visible lumps of chocolate. Remove from the heat and allow it to cool slightly for about 5-10 minutes – it should still be warm to the touch but not piping hot, so the eggs don't scramble in the next step.

Whisk Eggs and Sugars: In a separate large bowl, whisk together the caster sugar, light muscovado sugar, and the four eggs until the mixture is pale and thick, almost like a light, aerated mousse. This usually takes about 3-5 minutes with an electric whisk. You'll notice it has lightened in colour and increased in volume significantly. Stir in the vanilla extract at this stage.

Combine Wet Ingredients: Pour the slightly cooled chocolate and butter mixture into the egg and sugar mixture. Fold it in gently using a spatula until just combined. Be careful not to overmix; you want to retain the air you incorporated into the eggs. The mixture will be a rich, deep brown and beautifully smooth.

Add Dry Ingredients: Sift the plain flour, cocoa powder, and sea salt flakes directly over the chocolate mixture. Gently fold these dry ingredients in until there are no visible streaks of flour. Again, avoid overmixing. If you're adding chocolate chips or nuts, fold them in now. The batter will be thick and shiny, a truly tempting dark brown colour.

Bake and Cool: Pour the batter into your prepared baking tin and spread it evenly with a spatula. Bake for 25-30 minutes. You'll know they're done when the edges look set and a bit cracked, but the centre still has a slight wobble when you gently shake the tin. A skewer inserted into the centre should come out with moist crumbs attached, not wet batter. The kitchen will smell gloriously of rich chocolate! Let them cool completely in the tin on a wire rack before attempting to cut them. This is crucial for that fudgy texture.