Prepare the Tin: Preheat the oven to 175°C (160°C fan). Butter the base and sides of an 8-inch square baking tin, then line it with baking paper, leaving plenty of paper overhanging the edges for easy removal.
Melt the Butter Mixture: Place the butter and 100 g of the sugar in a medium saucepan over medium heat. Stir slowly until the butter has completely melted and the sugar has dissolved, about 1–2 minutes. Remove from the heat and allow the mixture to cool slightly. Whisk the Eggs: Add the eggs and remaining 200 g of sugar to a large bowl. Using a hand-held electric mixer, whisk for about 5 minutes, until the mixture becomes light, smooth, and fluffy.
Combine the Wet Mixtures: Pour the slightly cooled butter and sugar mixture into the whipped egg mixture. Whisk until the ingredients are evenly combined.
Mix the Dry Ingredients: In a separate medium bowl, stir together the plain flour, cocoa powder, baking powder, and salt until evenly distributed.
Form the Batter: Add the dry mixture to the bowl containing the egg, sugar, and butter mixture. Whisk just until combined, taking care not to overmix; a few small streaks of flour are fine.
Add Chocolate and Cherries: Add the chopped chocolate and prepared cherries to the batter. Use a spatula to gently fold them through until they are evenly incorporated.
Fill the Tin: Transfer the brownie batter to the prepared baking tin and spread it into an even layer with a spatula. If desired, reserve a small handful of chopped chocolate and scatter it over the surface.
Bake the Brownies: Bake for 38–40 minutes, or until the top is set and shiny with a few cracks across the surface. A toothpick inserted into the center should come out with crumbs rather than clean.
Cool and Set: Leave the brownies in the tin for at least 1 hour to cool and set, or allow them to cool completely. For a firmer set, they can be transferred to the refrigerator and left overnight.
Slice and Serve: Use the overhanging baking paper to lift the brownies from the tin. Cut into 16 square pieces and serve.
