Preheat oven to 350 degrees F (175 degrees C). Grease a deep pie dish.
Place rhubarb into the bottom of the prepared pie dish and sprinkle with cinnamon. Mash the butter and sugar together in a bowl until the mixture is creamy and well blended; beat in eggs, milk, and vanilla extract with an electric mixer until the mixture is smooth. In a separate bowl, stir the flour and baking powder together; gently stir the flour mixture into the butter mixture. Spread the resulting batter over the rhubarb in the pie dish.
Bake in the preheated oven until the crust is lightly browned and the rhubarb is bubbling, about 45 minutes. Serve warm.