Preheat oven to 350F and lightly grease and flour a 13×9 baking pan.

In stand mixer, or using an electric mixer in large bowl, cream together butter and cream cheese.

Beat in sugars 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, medium-sized bowl, whisk together 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 thick).

Quickly whisk together sugar and cinnamon for cinnamon ripple layer in a small bowl and sprinkle it evenly over the batter.

Dollop remaining batter over the cinnamon sugar layer and use a knife to spread evenly overtop. If it gives you a tough time/doesn't want to spread, just try your best but don't stress! If it's easier, just swirl the cinnamon/sugar in with a knife. Set aside and prepare your streusel topping.

Melt butter in a medium-sized bowl on 10 second increments (stirring between) until just melted (you don’t want it to be too hot, if it does get too hot allow to cool.

Use a fork to stir together your flour, sugars, cinnamon, and salt.   Pour melted butter over mixture and use a fork to toss/claw together the ingredients until combined (mixture should be crumbly).

Sprinkle streusel evenly over batter.

Bake on 350F for 40-50 minutes (check doneness with a toothpick inserted in center, it should come out clean or with a few moist crumbs).

Allow to cool before cutting and serving.

If desired, make glaze by whisking together powdered sugar and milk.  Drizzle over cooled coffee cake.