• Preheat oven to 350°F. Grease and line 9x9 0r 8x8 cake pan with parchment paper.

Make the Streusel

• In a medium bowl, combine the flour, white sugar, brown sugar, and cinnamon.
• Cut in the butter with a pastry cutter until the mixture resembles coarse oatmeal. Set aside.

Make the Coffee Cake

• Using a hand mixer or stand mixer fitted with the paddle attachment, cream the softened butter and white sugar together until smooth.
• Add the sour cream and mix until combined.
• In a separate bowl, whisk together the eggs and buttermilk. Stir in the vanilla extract.
• In another bowl, whisk together the flour, baking powder, salt, and nutmeg.
• Alternately add the wet and dry ingredients to the butter mixture, beginning and ending with the dry ingredients. Mix just until combined. Do not overmix.
• Grease a 9x9-inch baking pan. For easier removal, line the pan with parchment paper, leaving some overhang.
• Pour half of the cake batter into the prepared pan and gently spread into an even layer.
• Sprinkle half of the cinnamon streusel evenly over the batter.
• Spoon the remaining batter over the streusel and gently spread it out.
• Sprinkle the remaining streusel mixture evenly over the top.
• Bake at 350°F for30 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
• Place the pan on a wire rack and allow the cake to cool before slicing.
• Optional Blueberry Variation
• Add 1 cup fresh or frozen blueberries over the first layer of batter along with the middle layer of streusel. If using frozen blueberries, the cake may require additional baking time.