Preheat the oven to 180°C and line a 20x26cm tin with parchment.
Melt butter and white chocolate in a bowl over simmering water until smooth.
Whisk sugar, eggs, and vanilla for several minutes until pale and moussey.
Fold the melted chocolate, flour, and chopped dark chocolate into the egg mixture.
Pour into the tin, scatter glacé cherries over, and bake for 30 minutes.
Heat cream and milk until almost boiling, then whisk into a mixture of yolks, sugar, and flour.
Cook the custard with dark chocolate on low heat until thickened and smooth.
Strain the custard and cool completely with cling film on the surface.
Layer blondie cubes in a bowl, add Kirsch, drained cherries, and jam.
Spoon the custard over and chill in the fridge for several hours until set.
Whip mascarpone and cream to soft peaks and spread over the custard.
Garnish the top with grated chocolate before serving.