In a bowl, combine all-purpose flour, cocoa powder, granulated sugar, and salt. Cut in the cold unsalted butter until the mixture resembles coarse crumbs. Gradually add cold water until the dough holds together, then roll out and press into mini tart pans. Prick the bottoms and refrigerate until firm.

Preheat your oven to 350°F (175°C). Bake the tart shells for about 15 minutes, until they feel firm but not browned. Let them cool completely.

Heat heavy cream in a saucepan over medium heat until just simmering. Remove from heat and pour over chopped dark chocolate. Let sit for 2 minutes, then stir until the mixture is smooth and glossy.

In a separate bowl, whisk the eggs. Slowly incorporate the warm chocolate mixture into the eggs, whisking continuously to avoid cooking the eggs.

Pour the luscious chocolate filling into the cooled tart shells. Bake the filled tarts for an additional 10-12 minutes, until they are slightly set but still have a gentle jiggle in the center.

Allow the mini tarts to cool completely at room temperature before serving.