Gather all necessary ingredients. Make sure your butter is softened for easier mixing.

In a large mixing bowl, combine all-purpose flour, cocoa powder, powdered sugar, and salt. Whisk together until fully integrated.

Add softened unsalted butter and egg yolk to the dry mixture. Use a pastry cutter or hands to combine until a dough forms.

Press the dough into the bottom and up the sides of a tart pan evenly, ensuring there are no gaps.

Preheat your oven to 350°F (175°C). Bake the crust for 15 minutes and allow it to cool completely.

In a saucepan, combine heavy cream and sugar. Heat over medium until simmering, stirring occasionally.

Remove from heat, then add chopped dark chocolate and vanilla extract. Stir until the chocolate is melted and the mixture is smooth.

Pour the chocolate filling into the cooled tart shell, smoothing the top with a spatula.

Cover the tart lightly and refrigerate for at least 2 hours to set properly.

Once set, remove from the refrigerator and carefully slice into pieces.

Serve chilled, optionally topped with fresh berries, whipped cream, or chocolate shavings.