Heat oven to 400 degrees.

Grease an 8" x 8" glass baking dish with butter and then set aside.

Whisk together flour, 2 tablespoons sugar, baking powder, and salt in a medium bowl.

Add 4 tablespoons butter and banana to the flour mixture and mix with a pastry blender.

Mix together milk and vanilla in a small bowl and stir until just combined.

Transfer dough to the prepared baking dish and spread over the bottom of the dish.

Arrange rhubarb in a single row, snaking around the edges until the entire top of the dough is covered.

Squeeze in as many pieces of rhubarb as possible.

Combine remaining sugar and cinnamon and then sprinkle over rhubarb.

Dab remaining butter and over rhubarb.

Bake until browned and bubbly, about 20 minutes.

Let cool slightly before serving.