1. Preheat oven to 350 and prepare a 9×13 pan with nonstick cooking spray or coat with melted butter.

Cinnamon Filling

1. Mix brown sugar, flour and cinnamon together and set aside.

Crumb Topping

1. In a separate bowl combine melted butter, sugar, cinnamon and flour to create a crumb mixture. Use your hands to create large crumb pieces.

Cake

1. For the cake, mix the butter, sugar, and brown sugar in the bowl of your stand mixer fitted with the paddle attachment on medium speed for 2 minutes until light and fluffy. 
2. Add in the eggs, vanilla, sour cream (or greek yogurt), salt, and baking powder and mix for 1 minute until combined and smooth, scraping the sides of the bowl as necessary. 
3. Turn the mixer to low and alternate adding in the flour and milk, beginning and ending with flour. Mix until just combined and smooth, again, scraping the sides of the bowl as necessary.
4. Spread half of the cake batter into the prepared pan. Sprinkle the cinnamon filling evenly on top of the batter. Carefully spread the remaining cake batter on top of the cinnamon layer.  Evenly sprinkle the crumb topping on top.
5. Bake for 45-55 minutes, or until the cake is set and a toothpick inserted in the center comes out clean.
6. Remove cake from oven and place pan on a wire rack to cool completely.
7. Whisk together the powdered sugar and milk until smooth and drizzle on top of the cooled cake. Or, if you would rather you could sprinkle with powdered sugar instead of making the icing (or leave them both off). This coffee cake is fabulous plain.