In a medium saucepan, combine 1 cup of sugar, 2 tablespoons of flour, 1 teaspoon of ground cinnamon, and a pinch of salt. Stir in 1 cup of water and add 2 tablespoons of butter. Bring the mixture to a gentle boil over medium heat for about 1 minute, stirring constantly until thickened. Remove from heat and mix in 1 teaspoon of vanilla extract. Set aside while you prepare the dumplings.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1/4 cup of sugar, 1 tablespoon of baking powder, and 1/2 teaspoon of salt. Cut in 1/4 cup of cold butter using a pastry cutter until the mixture resembles coarse crumbs. Gradually add 3/4 cup of cold milk, mixing until just combined. Turn the dough onto a floured surface and roll it out into a 12x9-inch rectangle.

Spread 2 tablespoons of softened butter evenly over the rolled-out dough. Layer with 2 cups of chopped fresh or frozen rhubarb, then sprinkle with 1/2 cup of sugar and an additional 1 teaspoon of cinnamon. Starting from one long side, carefully roll the dough tightly into a log. Slice the rolled dough into 12 equal pieces and place them in a greased 9x13-inch baking dish.

Preheat your oven to 350°F (175°C). Once preheated, pour the prepared sauce evenly over the arranged dumplings in the baking dish. Bake for 35-40 minutes, or until the dumplings are golden brown on top and have puffed up nicely. They should be firm to the touch and lightly browned on the edges when done.