Preheat oven to 350F and lightly grease with cooking spray and flour a 13×9 baking pan.
In stand mixer, or using an electric mixer in large mixing bowl, cream together butter and cream cheese.
Beat in brown and white sugar until light and fluffy.
Add eggs, one at a time, beating until combined. Scrape down sides and bottom of mixing bowl with rubber spatula.
Stir in vanilla extract.
In a separate, large bowl, whisk together your dry ingredients: flour, baking powder, cornstarch and salt.
Gradually add flour mixture to wet ingredients, alternating with buttermilk, until all of the flour mixture and buttermilk have been combined.
Spread about half of the batter into prepared pan (batter will be very thick).
Spread the guava marmalade as evenly as possible on the bottom half of the batter.
Dollop remaining batter over the guava layer and use a spatula to spread evenly overtop.
Sprinkle streusel evenly over batter.
Bake on 350F for 40-50 minutes
Once out of the oven, allow to cool completely before cutting into it.
Top with a drizzle of cream cheese glaze and extra guava jam before serving