Heat 1 cup of whole milk and 4 tablespoons of unsalted butter in a saucepan until melted. Remove from heat, add yeast and sugar, stir and let sit for 5-10 minutes.

In a mixing bowl, combine the yeast mixture, 1 beaten egg, 3 1/4 cups all-purpose flour, and 3/4 teaspoon salt, mixing until a dough forms.

Knead the dough on a floured surface for 8-10 minutes, then shape into a ball and let it rise in a greased bowl for 1 to 1.5 hours.

Prepare the rhubarb filling by combining sliced rhubarb, sugar, lemon juice, and cornstarch in a saucepan and cook for 10 minutes until thickened.

Once the dough has risen, punch it down and roll it out into an 18x12 inch rectangle. Spread 1 cup of the rhubarb filling over the dough, roll tightly, and cut into 12 pieces.

Place the rolls in a greased baking pan, cover, and let them rise for another 30-45 minutes until puffy.

Preheat the oven to 350°F about 15 minutes before baking. Brush the rolls with a beaten egg and bake for 25-30 minutes until golden brown.

Prepare the glaze by whisking the remaining rhubarb filling, powdered sugar, and lemon juice together until smooth.

Allow the rolls to cool slightly, then drizzle with glaze and serve warm.