Prepare Your Tin: Preheat your oven to 180°C (160°C fan). Grease a 20cm (8-inch) square baking tin and line it with baking parchment, leaving some overhang on the sides to act as handles for easy removal later.

Melt Chocolate and Butter: In a medium, heatproof bowl set over a saucepan of gently simmering water (a bain-marie), melt the butter and chopped dark chocolate together. Stir occasionally until smooth and glossy. You can also do this in the microwave in 30-second bursts. Once melted, remove from the heat and set aside to cool for about 5-10 minutes.

Whisk Eggs and Sugar: In a separate large bowl, using an electric hand mixer or a stand mixer with the whisk attachment, beat the caster sugar and eggs together on medium-high speed for 3-4 minutes. The mixture should become pale, thick, and significantly increase in volume. Whisk in the vanilla extract.

Combine Wet Ingredients: Gently pour the cooled chocolate-butter mixture into the whisked eggs and sugar. Fold it in carefully with a spatula until just combined. Next, add the Greek yoghurt and fold again until the batter is uniform.

Add Dry Ingredients: Sift the plain flour, cocoa powder, baking powder, and salt directly into the wet mixture. What works best for me is to fold these in with a large metal spoon or spatula in gentle figure-of-eight motions. Stop mixing as soon as you no longer see streaks of flour. It's crucial not to over-mix at this stage.

Fold in Chocolate Chips: If using, gently fold in the 100g of chocolate chips or chunks.

Bake to Perfection: Pour the batter into your prepared tin and smooth the top with your spatula. Bake for 25-30 minutes. The top should have a shiny, crackly crust and the edges should be set. To check for doneness, a skewer inserted into the centre should come out with moist, fudgy crumbs attached, not wet batter.

Cool Completely: This is the hardest part! Allow the brownies to cool completely in the tin on a wire rack. This allows them to set properly, ensuring a dense, fudgy texture. Once cool, use the parchment handles to lift them out and slice into 16 squares.