Preheat and Prep: First, preheat your oven to 180°C (160°C Fan/Gas Mark 4). Grease and line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang on the sides to act as handles. This will make it much easier to lift the brownies out later.

Melt Chocolate and Butter: Place the cubed butter and chopped dark chocolate in a heatproof bowl. Set it over a small saucepan of simmering water (a bain-marie), making sure the bottom of the bowl doesn't touch the water. Stir occasionally until everything is completely melted and smooth. Alternatively, you can melt them in the microwave in 30-second bursts, stirring in between. Set aside to cool slightly.

Whisk Eggs and Sugar: In a large bowl, using a stand mixer or an electric hand mixer, whisk the eggs and caster sugar together on high speed for about 5-7 minutes. What works best for me is to keep going until the mixture is pale, very thick, and has at least doubled in volume. When you lift the whisk, it should leave a ribbon-like trail on the surface for a few seconds.

Combine Wet Ingredients: Gently pour the slightly cooled chocolate-butter mixture into the whisked egg mixture. Add the vanilla extract and carefully fold everything together with a spatula until just combined. Be gentle here to avoid knocking out all the air you just incorporated.

Sift and Add Dry Ingredients: Sift the plain flour, cocoa powder, and salt directly into the wet mixture. This prevents lumps and keeps the batter light.

Fold the Batter: Using your spatula, gently fold the dry ingredients into the wet ingredients until you can see just a few floury streaks remaining. It's crucial not to overmix at this stage; overmixing develops gluten and can lead to tough, cakey brownies instead of fudgy ones.

Add Chocolate Chips: Tip in the 100g of chocolate chips and give the batter a final few folds to distribute them evenly and incorporate the last of the flour.

Bake to Perfection: Pour the glossy batter into your prepared baking tin and smooth the top with your spatula. Bake on the middle shelf of the preheated oven for 25-30 minutes. The top should have formed a shiny, crackly crust, and the edges should be slightly set.

Check for Doneness: To check if they're ready, insert a skewer or cocktail stick into the centre. It should come out with moist, sticky crumbs attached, but not wet batter. This is the sign of a perfectly fudgy brownie.

Cool Completely: This is perhaps the hardest step! You must let the brownies cool completely in the tin on a wire rack. They will be quite soft when they first come out of the oven and need this time to set properly. Cutting them while warm will result in a delicious, gooey mess. Once cool, use the parchment paper handles to lift the brownie slab out and cut it into 16 squares.