Spray an 8" x 8" cake pan with baking spray, or grease with butter and sprinkle lightly with flour. Tap out excess flour. Preheat oven to 325 degrees.
Mix room-temperature butter and sugar in a large bowl using a hand mixer until light and fluffy. Do not over-mix.
Add eggs, one at a time. Mix just to incorporate the ingredients.
Add the flour, baking powder, baking soda, and salt. Mix on low until the batter is well mixed. Scrape down the sides of the bowl with a rubber spatula.
Add the Greek yogurt and vanilla. Mix until combined. Don't over-mix.
Pour half of the batter into the bottom of the prepared pan. Spread it out evenly.
In a medium bowl add the grated butter, brown sugar, salt, cinnamon, and flour.
Mix with a fork or your fingertips until crumbly. You should still see little bits of butter.
Sprinkle half the streusel mixture evenly over the top of the batter.
Add the rest of the batter onto the cinnamon layer in large dollops.
Use a knife or offset spatula to spread the dollops into an even layer.
Sprinkle the remaining half of the crumb topping mixture onto the batter.
Bake in a 325-degree oven for 35 mins or until a toothpick comes out clean.
Cool the coffee cake before slicing. I like to cut the cake in thirds and again in thirds in the opposite direction for 9 generous slices of cake. Best served warm or at room temperature.