Preheat your oven to 350°F (175°C). In a mixing bowl, combine flour, cocoa powder, and sugar. Cut in softened butter until the mixture resembles coarse crumbs.

Press the mixture into a tart pan evenly across the bottom and up the sides. Prick with a fork and bake for about 15 minutes until set.

Melt dark chocolate in a heatproof bowl over simmering water or in short bursts in the microwave until smooth; let cool slightly.

In another bowl, whisk eggs and sugar until pale and fluffy. Gradually mix in heavy cream, then fold in melted chocolate.

Pour the filling into the cooled tart shell and bake for an additional 20-25 minutes until just set with a slight jiggle.

Allow to cool completely at room temperature before chilling in the refrigerator for at least an hour.