Preheat the oven to 160ÆC fan/180ÆC/gas 4. Measure the olive oil, sugar and chocolate into a saucepan, then stir over a low heat until the chocolate has melted and the mixture is glossy.
Stir in 70ml of the milk, then let the mixture cool for 5–10 minutes before whisking in the eggs.
Stir into the liquid mixture the cocoa powder, ground almonds, sea salt and half the jarred cherries.
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 160ÆC fan/180ÆC/gas 4. Store any leftovers in the fridge.