Preheat oven to 350F and lightly grease and flour a 13x9 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. It's OK if you end up disturbing the cinnamon sugar layer a bit, it will be a nice swirl when it bakes. 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 whisk together your flour, sugars, cinnamon, and salt. Pour melted butter over mixture and use a fork to stir 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).
Allow to cool before cutting and serving.
If desired, make glaze by whisking together powdered sugar and milk. Drizzle over cooled coffee cake.
Store leftover coffee cake in an airtight container at room temperature for up to 3 days.