Warm 1 cup of whole milk and 4 tablespoons of unsalted butter in a saucepan over medium heat until melted. Remove from heat, let cool until warm (about 110°F). Sprinkle in 2 teaspoons of instant yeast and 2 tablespoons of granulated sugar, stirring lightly. After 5 minutes, whisk in 1 large egg, then mix in 4 cups of all-purpose flour and 1 teaspoon of salt until a soft dough forms.

Transfer the dough to a floured surface and knead for about 8-10 minutes until smooth and elastic. Shape into a ball, place in a greased bowl, cover with a towel, and allow to rise until doubled in size, about 1 to 1.5 hours.

In a medium saucepan, combine 2 cups of sliced rhubarb, 1 cup of granulated sugar, 2 tablespoons of lemon juice, and 2 tablespoons of cornstarch. Cook over medium heat, stirring frequently until the mixture thickens, about 10 minutes. Remove from heat and let cool.

After the dough has risen, punch it down and roll it out on a floured surface into a ¼ inch thick rectangle. Spread the cooled rhubarb filling evenly over the dough, then roll tightly into a log. Cut into 12 pieces and place cut-side up in a greased baking pan.

Cover the pan with a towel and let rolls rise in a warm place for 30-45 minutes until puffed up.

Preheat oven to 350°F (175°C). Brush tops with egg wash and bake for 25-30 minutes until golden brown.

Mix 1 cup of powdered sugar with reserved rhubarb filling and 1 tablespoon of lemon juice until smooth. Drizzle over warm rolls.