Preheat the oven to 350℉.
To start, cream together the butter and the cream cheese. Then, add in the sugars and vanilla extract. Next, beat in the eggs, then add in the milk.
In a separate bowl combine the flour, baking powder, cornstarch and salt. Combine the dry and wet, the batter will be thick.
Now, fold in the rhubarb and spread in the baking dish.
Mix together the ingredients for the crumbled topping then spread across the rhubarb coffee cake batter in the baking dish.
Place in the preheated oven for 40-50 minutes or until you can insert a toothpick in the center and it comes out clean.
Allow the coffee cake to fully cool, then serve and enjoy.