Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.

In a large bowl, whisk together flour, 2 tablespoons sugar, baking powder, and salt.

Cut in 1/4 cup cold butter until the mixture resembles coarse crumbs, then stir in milk until a soft dough forms.

Roll the dough out on a floured surface into a 12x9 inch rectangle.

Spread the chopped rhubarb over the dough, then sprinkle with 1 cup sugar and cinnamon.

Roll up the dough tightly from the long side and cut into 12 equal slices.

Place the slices in the prepared baking dish.

In a small bowl, combine boiling water, 1/2 cup sugar, and 2 tablespoons butter; stir until sugar is dissolved and pour over the rolls.

Bake for 35 minutes or until the rolls are golden brown and the sauce has thickened.