Preheat the oven to 170C/150C Fan/Gas 3. Grease a 30x23cm/12x9in baking tin with butter then line the base and sides with baking paper.
Heat the plain chocolate and butter in a saucepan over a low heat until just melted and well combined, stirring occasionally. Remove from the heat and set aside to cool slightly (at least 5 minutes).
Meanwhile, whisk the eggs with the sugar in a large bowl until thick, pale and creamy.
Whisk the cooled chocolate mixture into the egg mixture, then gently fold in the flour, baking powder and half the cherries until just combined.
Spoon the brownie mixture into the prepared tin, then scatter over the white chocolate and the remaining cherries.
Bake the brownies in the oven for 25-30 minutes, or until the surface is cracked and a skewer inserted into the centre of the brownies comes out with just a little mixture sticking to it. Remove from the oven and set aside to cool completely in the tin, on a wire rack.
To serve, dust the brownies with cocoa powder, then cut into squares and remove from the tin.