For the Sponge: Preheat the oven to 200 degrees celsius. Make sure the butter is soft before adding it to the mixing bowl, along with the butter and eggs. Mix on a slow setting, until a smooth consistency. Once smooth, add the peanut butter and natural yogurt. When everything has been mixed, slowly add in the flour, adjusting the speed to a moderate setting. When all the flour has gone in, pour the mixture into a brownie tray (approx 20cmx8cm) and bake for 15 - 20 mins, until golden. Remove from oven and place on a cooling rack.