Sift 150 g plain flour and 50 g Dutch-processed cocoa powder into a large mixing bowl. Next, add 80 g salted butter (cubed) and using your hands, rub it into the flour-cocoa mixture until it resembles breadcrumbs. Add 1 medium egg yolk and mix well. Finally, add 4 tablespoons water or milk* in increments and work together until the pastry just comes together in a ball. Don't knead the pastry; wrap it in reusable wraps (or clingfilm) and place in the fridge to chill for a minimum of one hour.
Once the pastry has chilled, remove it from the fridge. Flour your work surface, and using a rolling pin, roll the pastry out to just over nine inches in diameter. Carefully place it into a nine inch fluted tart tin, and push the pastry into the sides. Prick the surface with a fork, then chill in the fridge again, for a minimum of 30 minutes.
Blind Bake the Pastry
Preheat your oven to 180℃ (356℉).
Cover the pastry with parchment paper. Tip ceramic baking beans onto the parchment paper, until the tart is full. Blind bake for 20 minutes, then leave to cool.
Make the Blackberry Mascarpone Filling
Thoroughly wash 300 g blackberries. Add the berries to a small saucepan along with 2 tablespoons granulated sugar. Turn the heat to medium-high and cook until the mixture comes to a boil. It should reduce down and the berries should soften. Wait for this mixture to cool, then pass through a sieve.
To the bowl of a large stand mixer, add 100 ml double cream, 250 g mascarpone, 150 g cottage cheese, 60 g icing sugar, and ¼ teaspoon Madagascan vanilla extract. Whisk until the mixture reaches a soft-peak stage.
Fold the blackberry syrup into the cream mixture.
Assemble the tart.
Remove the baking beans and parchment paper from the cooled pastry.
Carefully spoon in the blackberry mascarpone filling. Level the surface. Optionally, decorate with extra blackberries, shaved dark chocolate, chocolate pieces, and edible flowers.
For the best texture, chilli overnight.
Notes
* Your pastry may need more or less liquid, depending on the quality of your flour. Always add in 1/2 tablespoon increments.