Step 1

In a small bowl whisk together the brown sugar, flour, and cinnamon then set aside.

Step 2

Add the brown sugar, salt, cinnamon, and flour to a medium bowl. Whisk together then add the diced butter. I usually use room temperature butter for this but you can use cold butter too.

Step 3

Use your clean fingers to press and crumble the butter into the dry mixture. Once you have a crumbly mixture you can set it aside. For larger crumbles try pressing a large handful together and break apart into large pieces.

Step 4

Preheat oven to 350F. Line a 9x9 inch pan with parchment paper, or grease it, and set aside. Sift the flour, cornstarch, salt, and baking powder into a large bowl, whisk together and set aside.

Step 5

Beat the butter and granulated sugar in a stand mixer fitted with a paddle attachment or a large bowl If using an electric hand mixer. Mix until light and fluffy then add the brown sugar and mix on high. Pour in the vanilla then add the eggs one at a time while mixing on low. Scrape the bowl down then add the sour cream and mix until combined.

Step 6

Add the dry mixture to the wet and mix until just combined then use a spatula to scrape the bowl down and mix any stray bits in.

Step 7

Pour half the cake batter into the prepared pan and smooth out. Sprinkle the cinnamon filling on top. Spread the remaining batter over the cinnamon filling moving from the center to the edges. Top with the streusel and bake at 350F for about 50-55 minutes or until a toothpick inserted in the center comes out clean. Allow to cool in the pan for 20 minutes before cutting.