In a food processor, pulse together all-purpose flour, Dutch process cocoa powder, powdered sugar, and fine salt until blended. Add cold, cubed unsalted butter and process until the mixture resembles coarse crumbs. Incorporate an egg yolk and pulse until a dough forms.

Shape the tart dough into a disc, wrap it tightly in plastic wrap, and refrigerate for 40-60 minutes. Preheat your oven to 350°F (175°C).

On a lightly floured surface, roll out the chilled dough to fit your tart pan. Carefully transfer the dough into the pan and press it gently into the corners. Use a fork to dock the bottom of the crust.

Line the crust with parchment paper and fill it with pie weights or dried beans. Blind bake in the preheated oven for 20 minutes, brush with egg white, and return for an additional 5 minutes.

Using a double boiler, melt together dark chocolate, milk chocolate, and a tablespoon of unsalted butter until smooth and glossy.

In a mixing bowl, whisk large eggs with caster sugar until pale and fluffy. Fold in the melted chocolate mixture, then sift in remaining dry ingredients.

Spread cherry jam over the cooled tart shell, then pour the chocolate filling over it, scattering fresh pitted cherries on top.

Bake the assembled tart for 20-25 minutes, until the filling is set with a slight wobble in the center. Let cool completely.

In a mixing bowl, whip heavy cream, mascarpone cheese, and a touch of caster sugar until stiff peaks form.

Top the cooled tart with whipped cream and additional fresh cherries. Slice into portions and serve.