In separate, medium-sized bowl, whisk together flour, baking powder, cornstarch and salt. Set aside.
Using a mixer, in a large bowl, cream together butter and softened cream cheese.
Add in sugars and beat until light and fluffy.
Add eggs, one at a time, beating until combined.
Stir in vanilla extract.
Add flour mixture to wet mixture, alternating with some of the buttermilk several times mixing with each addition until all combined.
Spread almost half of the thick batter into prepared pan.
Sprinkle the CINNAMON SUGAR mixture evenly over the batter.
Spoon remaining batter over the cinnamon sugar layer and use a knife to spread evenly over the top. You can also swirl the batter to get more cinnamon swirls, optional.
Sprinkle STREUSEL TOPPING evenly over batter.
Bake on 350F for 40-50 minutes. Check doneness with a toothpick.
Allow to cool before cutting and serving.