Roll out the pastry until roughly 3mm thick and line a 30cm (12 inches) loose bottomed tart tin. Prick the base with a fork and refrigerate for 30 minutes.
Begin by making the pastry. Place the flour and cocoa in a food processor fitted with a chopping blade and process to combine. Add the butter and pulse until the mixture resembles fine breadcrumbs. Add the yoghurt and continue to pulse until the dough starts to form a ball. Remove from the food processor and wrap in plastic wrap and refrigerate for 30 minutes.