Warm 1 cup of whole milk and melt 1/2 cup of unsalted butter together in a saucepan. Whisk in 2 1/4 teaspoons of instant yeast and 1/4 cup of granulated sugar, letting it proof for about 5 minutes until frothy.

In a stand mixer, combine the proofed yeast mixture with 1 large egg, 4 cups of all-purpose flour, and 1 teaspoon of salt. Knead on medium speed for about 8 minutes until the dough is smooth and elastic.

Shape the dough into a ball and place it in a greased bowl. Cover with a kitchen towel and let it rise in a warm place until doubled in size, about 1 to 1.5 hours.

In a saucepan, combine 2 cups of sliced rhubarb, 1/2 cup of granulated sugar, 2 tablespoons of lemon juice, and 2 tablespoons of cornstarch. Cook over medium heat for about 10 minutes until thickened into a jam-like consistency; cool slightly.

Punch down the risen dough and roll it out into an 18x12 inch rectangle on a floured surface. Spread 1 cup of cooled rhubarb filling over the dough, leaving the edges bare.

Roll the dough tightly from one long side to the other, then cut into 12 equal pieces. Arrange these in a greased baking pan, cut side up.

Cover the shaped rolls with a kitchen towel and let them rise for another 30 to 45 minutes until puffy.

Preheat your oven to 350°F (175°C). Brush the rolls with a beaten egg mixed with 1 tablespoon of water or milk.

Bake in the preheated oven for 25 to 30 minutes, or until golden brown on top.

In a bowl, mix the remaining rhubarb filling with 1 cup of powdered sugar and 1 tablespoon of lemon juice; pour this glaze over the warm rolls once they come out of the oven.