Preheat oven to 350 and butter an 8×8 pan. Combine the butter and sugar until smooth, then add in the other wet ingredients (except the yogurt) and stir. Add the dry ingredients and mix till smooth, then stir in the yogurt. Spread into the prepared pan and set aside. In another bowl combine the topping ingredients to form grainy crumbles, and spread that evenly over top of the cake batter. Bake for 40 to 45 minutes until a toothpick in the center comes out clean. Allow it to cool for five or ten minutes and then top with glaze and serve warm, or cover and store at room temperature for up to three days.