Line a 900g loaf tin or terrine with baking parchment. Tip the cherries into a sieve over a saucepan and drain off the syrup. Bring the syrup to a boil and simmer until it thickens and is reduced by half, about 5 mins, and stir back in the cherries. Spoon the cherries and syrup into the base of the tin and leave to cool.
Whip half of the cream until it thickens but is not stiff, spoon this in a layer over the cherries. Cut the brownie or cupcake into pieces and make a layer on top of the cream. Melt 100g chocolate carefully in a small pan and leave to cool a little.
Tip the mascarpone into a bowl and add the remaining cream and the icing sugar, whisk together with electric beaters until they are just combined and the mixture is thick, stir in the melted chocolate with a spatula. Spoon the chocolate mixture on top and smooth the surface. Put the trifle into the fridge for 30 mins.
Melt the remaining chocolate in the same way, leaving to cool a little and pour it onto the trifle, tip the loaf tin back and forth to make a thin, even layer. Wait for the chocolate to set and then cover the tin or terrine and put it into the freezer. Freeze for up to 1 week.
To serve, take the tin or terrine out of the freezer and leave it to sit for 10 mins, then tip it out onto a serving plate. The syrup from the cherries will run down the sides a little, leave the trifle to sit for another 20 mins before cutting into slices to serve.