1teaspoonunsweetened cocoa powder (optional, added for deeper color not flavor)
Preheat oven to 350°F. Spray a 9x9 inch square pan with cooking spray. Set aside.
To make the streusel: In a small bowl, mix flour, granulated sugar, brown sugar, cinnamon and salt until well combined. Add the melted butter and mix well until crumbs form. Set aside.
To make the cake: In a medium bowl sift together flour, baking powder, baking soda and salt. Set aside.
In a mixer bowl, beat the softened butter on medium speed for one minute until creamy. Add sugar and beat 30 seconds. Add eggs, and beat 3 minutes or until batter is light and fluffy, scraping the sides of the bowl as necessary. Mix in vanilla extract. With mixer on low speed, mix in the flour mixture in 3 additions, alternating with the sour cream in 2 additions (starting and ending with the flour). Do not overmix. Batter should be thick and fluffy.
Spoon half of the batter into the prepared pan and smooth it into an even layer. Sprinkle the filling mixture evenly over the batter. Spoon the remaining batter on top and smooth it into an even layer. Sprinkle the streusel mixture evenly on top of the cake, gently patting down.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean. There may be crumbs but there shouldn’t be batter on toothpick. Place the cake in the pan on a wire rack to cool completely.
Serve warm, or at room temperature. If desired, serve with whipped cream or ice cream.
Cake will keep 1-2 days at room temperature. To freeze, wrap very well, and freeze for up to 3 months. Allow to thaw about 90 minutes before using.
If desired, add a heaping half cup of chopped nuts such as walnuts or pecans to the streusel mixture before baking.