Prepare Your Baking Tray: Preheat your oven to 175°C (160°C fan/Gas Mark 3). Grease and line a 23cm x 23cm baking tray with parchment paper, leaving an overhang on two sides to help lift the brownies out later.

Melt the Chocolate and Butter: In a large heatproof bowl set over a saucepan of simmering water (a double boiler), combine the cubed unsalted butter, dark chocolate, and milk chocolate. Stir gently until the mixture is smooth, glossy, and beautifully melted. There should be no visible lumps, and the colour will be a deep, rich brown. Remove from the heat and set aside to cool slightly for about 5-10 minutes.

Whisk Eggs and Sugars: In a large mixing bowl, using a stand mixer with the whisk attachment or a hand mixer, beat the caster sugar, light brown sugar, and eggs together on high speed for 5-7 minutes. The mixture will transform into a pale, thick, and voluminous foam, almost like a light mousse, and will have ribbons forming on the surface when the whisk is lifted. This aeration is key for that fudgy texture.

Combine Wet Ingredients: Gently fold the slightly cooled melted chocolate mixture into the egg and sugar mixture using a rubber spatula. Mix until just combined; be careful not to deflate the airy egg mixture too much. Stir in the vanilla extract until its sweet aroma permeates the batter. The mixture will be a smooth, dark, and shiny batter.

Fold in Dry Ingredients: Sift the plain flour, cocoa powder, and salt together directly over the wet ingredients. Carefully fold these dry ingredients into the chocolate batter using your spatula until just combined. Stop mixing as soon as you no longer see streaks of flour. Overmixing will develop the gluten and result in a cakey rather than fudgy brownie. The batter will be thick, dark, and rich.

Bake the Brownies: Pour the brownie batter into your prepared baking tray and spread it evenly with the spatula. Bake for 30-35 minutes. You'll know they're ready when the edges look set and a little cracked, but the centre still has a slight wobble when gently shaken. A skewer inserted into the centre should come out with moist crumbs attached, not wet batter. The smell of rich chocolate will fill your kitchen!

Cool Completely: Once baked, remove the brownies from the oven and let them cool completely in the tray on a wire rack. This is crucial for achieving the perfect fudgy texture and clean cuts. Resist the urge to cut them warm! They will feel soft to the touch, and as they cool, they will firm up beautifully.

Prepare the Chocolate Ganache: While the brownies are cooling, finely chop your ganache dark chocolate and place it in a heatproof bowl. In a small saucepan, gently heat the double cream until it just begins to simmer around the edges – do not boil. Pour the hot cream over the chopped chocolate and let it sit for 2-3 minutes without stirring. Then, add the tablespoon of butter and stir gently with a whisk or spatula from the centre outwards until the mixture is completely smooth, glossy, and forms a thick, flowing sauce. It will be a deep, luxurious brown.

Assemble and Decorate: Once the brownies are completely cool, pour the prepared chocolate ganache over the top, spreading it evenly with an offset spatula or the back of a spoon to create a smooth, reflective surface. If desired, immediately scatter your chosen coloured sprinkles over the wet ganache. Allow the ganache to set at room temperature for at least 30 minutes, or in the fridge for 15-20 minutes, until it's firm to the touch.

Slice and Serve: Once the ganache is set, use the parchment paper overhang to lift the entire brownie slab out of the tray. Using a sharp knife, warmed under hot water and wiped clean between each cut, slice the brownies into 16 squares. The knife will glide through the ganache and brownie, revealing neat, fudgy squares.