Sift the flour, cocoa powder and icing sugar into the bowl of a food processor, add the butter and a pinch of sea salt, then pulse until it resembles breadcrumbs.
Add the egg yolk and 2 tablespoons of iced water and pulse again, until the dough comes together.
Tip out onto a floured surface and bring it together with your hands. Wrap it in clingfilm and chill for 20 minutes.
Roll out the pastry so it’s large enough to line a 22cm loose-bottomed tart pan. Chill for 15 minutes.
Line the pastry with baking paper and fill with baking beans. Bake for 8 minutes, then remove paper and beans and bake for a further 5 minutes until dry. Allow to cool slightly.
Place the eggs and sugar in a bowl and sift in the cornflour, cinnamon and cocoa. Gently whisk together until combined, but not frothy. Add the wine and 200ml cream, and stir well until combined.
Spread a layer of cherry jam onto the bottom of the tart shell and then pour the chocolate filling over the top - you may find this easier to do near the oven so you don’t have to carry the filled tart case very far!
Bake the tart for 20-25 minutes or until just set, then remove from the oven and set aside to cool slightly.
Meanwhile, whisk the remaining 100ml cream to soft peaks.
Dust the top of the tart thickly with cocoa, then slice and serve with the whipped cream.