In a mixing bowl, combine all-purpose flour, Dutch process cocoa powder, powdered sugar, and fine salt. Add cold, cubed unsalted butter and mix until resembling coarse crumbs. Incorporate one egg yolk to form a crumbly dough. Shape into a disc.

Wrap the dough disc tightly in plastic wrap and refrigerate for 40-60 minutes.

Preheat your oven to 350°F (175°C). Roll out the chilled dough to fit your tart pan, transfer into the pan, and dock the base with a fork.

Line tart shell with parchment paper and fill with pie weights. Bake for 20 minutes. Remove weights, brush base with egg white, then bake for another 5 minutes.

Set up a double boiler and melt dark chocolate, milk chocolate, and cubed unsalted butter until smooth.

In a separate bowl, whisk eggs and caster sugar until pale and fluffy. Fold in melted chocolate mixture and sift in cocoa powder.

Spread cherry jam over cooled tart shell, pour in chocolate filling, and nestle fresh pitted cherries evenly.

Bake for 20-25 minutes until edges are set and center is slightly wobbly. Let cool completely.

Whip heavy cream with mascarpone cheese and sugar until stiff peaks form. Optionally whisk in vanilla extract.

Top tart with whipped cream and garnish with additional fresh cherries. Slice and serve immediately.