For sweet pastry, process flour and butter in a food processor until fine crumbs form, then add egg and ½ tablespoon water and process until just combined. Turn onto a lightly floured work surface and knead lightly until dough is smooth, then flatten into a disc, wrap in plastic wrap and refrigerate until firm (45 minutes-1 hour).
Preheat oven to 190C. Roll out pastry to 3mm thick and line a buttered and floured 28cm-diameter straight-sided flan ring placed on a baking paper-lined oven tray. Trim edges, prick base using the tines of a fork and refrigerate for 30 minutes. Bake blind for 20 minutes, remove paper and weights and bake for another 10 minutes.
Meanwhile, combine cream, chocolate, butter and sugar in a heatproof bowl placed over a saucepan of simmering water, stirring occasionally until chocolate melts and mixture is smooth and glossy. In a separate bowl, whisk egg yolks to break down slightly, add 1 cup of chocolate mixture and whisk to combine. Add remaining chocolate mixture and stir gently to combine. (Don’t over-work as you’ll create air bubbles.) Reduce oven to 150C. Scatter prunes over tart base, then pour over chocolate mixture and bake until tart is just set (25-30 minutes). Cool to room temperature, then serve dusted with cocoa powder.