Preheat the oven to 350°F and grease a 9x13-inch baking pan. 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 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.