This is seriously the best coffee cake ever!! The cake itself has so much flavor and is super moist! It is slightly tangy from the Greek yogurt and buttermilk, just like classic coffee cake should be!! And the sweet cinnamon crumb topping is the perfect complement!
In a medium-sized bowl, combine the flour, brown sugar, and cinnamon. Add the melted butter and use a fork to mix the crumb topping together until no flour is visible. Set aside for later.
Preheat oven to 350ºF. Spray a 9 by 9 inch pan with nonstick spray. Set aside.
In a medium-sized bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, use a handheld electric mixer or stand mixer to beat the butter on medium speed for 2 minutes. Add sugar and beat on medium speed for 2 more minutes. Add egg and vanilla and mix until combined. Add half of the flour mixture and mix until mostly combined. Add the Greek yogurt and mix until incorporated. Add the rest of the flour mixture and beat until no lumps remain. Add the buttermilk and mix until smooth.
Pour the batter evenly into the prepared pan. Sprinkle the crumb topping evenly over the batter. Bake at 350ºF for 35-40 minutes, or until a toothpick inserted in the center of the cake comes out clean. Allow the cake to cool completely in the pan before cutting and serving.
Use 2% or full fat plain Greek yogurt.
Make Ahead Tip: Place foil over the top of the pan and store this cake at room temperature for 2-3 days.