Place the chocolate and butter into a large microwave-safe bowl and cook in the microwave for 30-60 seconds until fully melted. Stir to combine. Stir in the yoghurt and eggs until everything is totally blended.
Mix in the flour, cocoa powder, sugar, baking powder and salt until the mixture just comes together and there are no pockets of dry ingredients. Pour on top of the graham cracker base and spread into the corners.
Bake in the preheated oven for 20 minutes. When cooked, remove from the oven and leave to cool slightly on a wire rack for 10 minutes.