Prepare the Sweet Potato: If you haven't already, cook your sweet potato. You can bake it whole at 200°C (180°C fan) for 45-60 minutes until very soft, or peel, cube, and steam it for 15-20 minutes. Once cooked, let it cool slightly, then mash it until completely smooth. Measure out 250g and set it aside.

Preheat and Prep: Preheat your oven to 180°C (160°C fan). Grease a 20cm (8-inch) square baking tin and line it with parchment paper, leaving some overhang to help you lift the brownies out later.

Melt Chocolate and Butter: In a heatproof bowl set over a small saucepan of simmering water (a bain-marie), gently melt the butter and chopped dark chocolate together. Stir occasionally until smooth, then remove from the heat and let it cool for about 5-10 minutes.

Whisk Eggs and Sugar: In a separate large bowl, using an electric hand mixer or a stand mixer, whisk the eggs and light brown sugar together for 3-4 minutes. You're looking for the mixture to become pale, thick, and significantly increased in volume.

Combine Wet Ingredients: Gently pour the cooled chocolate-butter mixture into the whisked eggs and sugar. Add the sweet potato purée and vanilla extract. Fold everything together with a spatula until just combined.

Sift Dry Ingredients: Sift the plain flour, cocoa powder, baking powder, and salt directly into the wet mixture. This helps remove any lumps and aerates the flour.

Fold Gently: Using a spatula, gently fold the dry ingredients into the wet mixture until you can no longer see streaks of flour. What works best for me is to be very gentle here; over-mixing develops gluten and can lead to a cakey brownie.

Add Extras: If you're using them, fold in the chopped walnuts or chocolate chips now.

Bake: Pour the batter into your prepared tin and smooth the top with your spatula. Bake for 25-30 minutes. To check for doneness, insert a skewer into the centre – it should come out with a few moist, sticky crumbs attached. If it comes out clean, they're over-baked.

Cool Completely: Let the brownies cool completely in the tin on a wire rack. This is the hardest part! Slicing them while warm will result in a gooey mess. Once cool, use the parchment paper overhang to lift them out and cut them into 16 squares.