Preheat oven to 350° F (static) or 325° F (convection).
Oil and lightly flour (with gfJules Flour) or line with parchment one 8x8 pan or one 8x11 pan and set aside.
In a medium-size bowl, whisk together dry ingredients and set aside. In a large mixing bowl, beat butter and sugars until light. Add eggs one at a time, beating afterwards. Add yogurt, milk and vanilla, mixing to incorporate, then slowly stir in the dry ingredients, mixing until smooth, about 1-2 minutes only.
If making the filling, prepare that in a separate bowl.
Pour half the batter into the prepared baking dish then sprinkle filling on top and spread remaining batter on top of filling, spreading all the way to the sides of the pan. If not using filling, simply spread the batter into the pan evenly.
In a small bowl, stir together the topping ingredients with a fork until integrated. Crumble over the top of the batter and cut through slightly, with a knife pointed down, making criss-cross patterns into the top of the cake if you like the topping to marbleize. If using, lay apple slices across the top.
Bake in preheated oven for 55 minutes, then test with a toothpick for doneness -- it should only have dry crumbs when removed. When using the smaller 8x8 pan, the coffee cake will take longer to bake than in the larger pan, so add more time as needed until the toothpick comes out clean or only with dry crumbs.