Warm the Milk and Butter: In a small saucepan, heat 1 cup of whole milk and 4 tablespoons of unsalted butter over medium-low heat until warm. Whisk in 2 teaspoons of instant yeast and 2 tablespoons of granulated sugar. Let it sit for 5-10 minutes until bubbly.

Prepare the Dough: In a stand mixer bowl, combine the yeast mixture with 1 large egg, 4 cups of all-purpose flour, and 1 teaspoon of salt. Mix on low speed for about 5 minutes until smooth and elastic.

Let the Dough Rise: Transfer the dough to a greased bowl, cover, and let it rise in a warm area for 1 to 1.5 hours until doubled in size.

Cook the Rhubarb Filling: Combine 2 cups of sliced rhubarb, 1/2 cup granulated sugar, 2 tablespoons lemon juice, and 2 tablespoons cornstarch in a saucepan. Cook for about 10 minutes until thickened. Let cool slightly.

Roll Out the Dough: Once risen, punch down the dough and roll it out into a large rectangle, about 1/4 inch thick. Spread the rhubarb filling over it.

Shape the Rolls: Tightly roll the dough into a log, pinch seams to seal, and cut into 12 pieces. Place in a greased baking pan.

Second Rise: Cover the pan and let rolls rise for 30-45 minutes until puffy.

Preheat the Oven: Preheat the oven to 350°F (175°C). Optionally brush tops with egg wash.

Bake the Rolls: Bake for 25-30 minutes until golden brown and a toothpick comes out clean.

Prepare the Glaze: Mix 1 cup of powdered sugar with 2 tablespoons lemon juice until smooth.

Glaze the Rolls: Drizzle the glaze over warm rolls once baked.

Serve and Enjoy: Serve warm with coffee or tea.