Preheat the oven to 350°F and spray a 9×13-inch baking pan with cooking spray. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment combine butter, sugar, vanilla extract, and eggs. Mix for 3-4 minutes at medium speed until combined.
In a small bowl combine flour, baking powder, and salt.
With the mixer running at low speed, alternately add flour mixture and milk in two portions. Mix until just combined. Don’t overmix.
Pour batter into prepared pan and spread evenly. Top with sliced rhubarb and spread evenly.
To make the streusel, combine flour, sugar, vanilla extract, salt, cinnamon, and cubed butter in a bowl. Mix for 2-3 minutes until all the flour is incorporated. Distribute the streusel evenly over the rhubarb.
Bake for about 45-50 minutes, or until a toothpick comes out clean. Let cool then serve with whipped cream or vanilla ice cream.
The calories shown are based on the cake being cut into 20 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**