Preheat your oven to 350°F (180°C). Prepare a 9” (23cm) square pan by greasing it and lining it with parchment paper, allowing the paper to hang over the sides to create a 'sling.' You can use binder clips to secure the parchment if needed.

Melt the dark chocolate and butter together by placing them in a heatproof bowl over a pot of simmering water, ensuring the bottom of the bowl doesn’t touch the water. Stir frequently until smooth. Alternatively, you can microwave the mixture in 30-second intervals, stirring well between each until fully melted and combined.

Once the chocolate and butter mixture is smooth, sift the cocoa powder over it and stir until fully incorporated.

In a separate large bowl, whisk together the eggs, caster sugar, and brown sugar using an electric handheld mixer on medium-high speed. Continue mixing for 2 to 3 minutes, until the mixture becomes lighter in color and increases in volume. If you prefer, this step can be done with a stand mixer fitted with the whisk attachment.

Pour the melted chocolate mixture into the egg and sugar mixture, and blend until well combined. Add the salt and vanilla extract, mixing again to incorporate.

Gently fold in the sifted flour by hand, stirring until the batter is smooth and free of lumps.

Pour the brownie batter into your prepared pan, then tap the pan on the counter to release any trapped air bubbles. Let the batter settle for a minute or two before tapping again to ensure a smooth surface.

Transfer the 200g of creamy peanut butter into a piping bag, cutting off the tip to create an opening. Pipe small 'sausages' of peanut butter, cutting them into 2cm chunks, and distribute them evenly over the top of the brownie batter. If you don’t have a piping bag, you can use two teaspoons to drop the peanut butter onto the batter.

Using a spatula or spoon, gently cover the peanut butter chunks with the brownie batter, ensuring you don’t press them down too firmly.

Warm the remaining 50g of peanut butter slightly in the microwave in 10-second bursts or over simmering water. Drop spoonfuls of the warmed peanut butter over the surface of the brownie. Using a toothpick, knife tip, or cake tester, swirl the peanut butter into a decorative pattern on top.

Bake the brownies for 22 to 28 minutes, checking them at the 20-minute mark. The edges and top should be set, and when you insert a skewer into the center, it should come out with a few moist crumbs attached. It’s better to underbake slightly rather than overbake.

Once done, remove the brownies from the oven and place the pan on a wire rack to cool completely. When cooled, lift the brownies out using the parchment paper sling and cut them into squares with a sharp knife.

Store any leftover brownies in an airtight container at room temperature for up to four days or in the refrigerator for up to a week.