Beat the mascarpone with the remaining 20ml of milk until smooth, and set aside.
Pour the batter into a lined 24.5cm x 17.5cm baking tin, and drop in teaspoons of the beaten mascarpone. Use the handle of the teaspoon to draw the mascarpone gently through the chocolate mixture, then scatter over the remaining cherries and the chopped walnuts.
Transfer the tin to the oven and bake for 20–25 minutes, until the top of the brownie looks just set. Let the brownie cool in the tin for 5 minutes before transferring it gently to a wire rack.
Serve warm: if made in advance, you can gently reheat in the oven for 5 minutes at 160°C fan/180°C. Store any leftovers in the fridge.
Recipe adapted from The Sweet Roasting Tin (Square Peg)