Preheat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper or lightly grease it.

In a medium bowl, whisk together plain flour, cocoa powder, baking powder, and salt. Set aside.

In a heatproof bowl, melt the chopped dark chocolate and butter together. You can do this in the microwave in 30-second intervals, stirring between each interval, or on a double boiler. Once melted, set aside to cool slightly.

In a large bowl, whisk the eggs, caster sugar, and Demerara sugar until well combined and slightly fluffy.

Add the melted chocolate mixture to the egg mixture, followed by the vanilla extract. Stir until smooth and fully incorporated.

Gradually add the dry ingredients to the wet mixture, folding gently until just combined.

Fold in the chocolate chips until evenly distributed.

Pour the batter into the prepared baking pan and spread it evenly.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out mostly clean, with just a few moist crumbs.

Allow the brownies to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.

Once cooled, cut into squares and enjoy!