Begin with the tart crust. In a bowl or food processor, combine flour, powdered sugar, and salt. Add cold cubed butter and pulse or cut in with a pastry cutter until the mixture resembles coarse crumbs. Add the egg yolk and a tablespoon of cold water. Pulse again until the dough just starts to come together. If it’s too dry, add a little more water.

Turn the dough out onto a lightly floured surface, form into a disc, and wrap in plastic. Chill for 30 minutes. Once chilled, roll out the dough and press it into the tart pan, trimming the edges. Prick the base with a fork and chill again for 15 minutes. This step helps prevent shrinking.

Preheat your oven to 375°F (190°C). Line the tart shell with parchment and pie weights or beans, and bake for 15 minutes. Remove the weights and bake for another 10 minutes, or until golden. Let it cool completely before adding any filling.

While the crust cools, prepare the blueberry layer. In a small saucepan, cook the blueberries, sugar, and lemon juice over medium heat until the berries burst and the mixture becomes syrupy, about 5–7 minutes. Stir in the cornstarch mixture and cook for another 1–2 minutes, until thickened. Let it cool slightly, then spread the compote over the cooled tart shell.

Now, prepare the ganache. Heat the cream in a small saucepan until it just begins to simmer. Remove from heat and pour over the chopped chocolate. Let sit for 2 minutes, then stir until smooth. Add the butter and vanilla extract and stir again until glossy.

Pour the ganache over the blueberry layer in the tart shell, smoothing the top with a spatula. Chill the tart for at least 2 hours to set.