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.
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.
Make the crumble filling by combining the flour, butter, cocoa, sugar and oats in a mixer fitted with a paddle attachment. Mix until the mixture resembles breadcrumbs. Refrigerate.
Preheat the oven to 180 celsius (350 Fahrenheit) and line the tart with baking paper and pastry weights or beans. Bake for 15 minutes. Remove the baking paper and pastry weights and bake for another 5 minutes or until the base is dry.
Make the filling by combining the plums, sugar, cinnamon and flour in a bowl and then arranging over the tart base. Top with the crumble mixture and bake for 35 to 40 minutes or until the juices start to bubble through the crumble topping.
The pastry recipe will make more than is required for this tart. Wrap up remaining pastry in plastic film and freeze.