Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.

Bring water to a boil in a small saucepan. Dissolve 1 cup sugar in boiling water, then add 3 tablespoons butter. Remove from heat; set syrup aside to cool.

Whisk flour, baking powder, and salt together in a large bowl. Cut in 1/2 cup butter. Stir in milk; mix lightly.

Roll out dough on a floured surface into a rectangle shape about 12 inches long.

Combine rhubarb, cinnamon, and remaining 1 cup sugar in another bowl; spread evenly over dough, leaving a 1/2-inch border. Roll dough tightly lengthwise; seal the seam. Cut into twelve 1-inch pieces. Arrange rolls in the prepared baking dish. Pour syrup over rolls.

Bake in the preheated oven until lightly browned, about 40 minutes.