Make the dough: In a large bowl, combine warm milk and yeast. Let sit for 5–10 minutes until foamy. Stir in sugar, melted butter, egg, and salt. Add flour gradually and mix until a soft dough forms. Knead on a lightly floured surface for 5–7 minutes, until smooth and elastic.

Let it rise: Place dough in a greased bowl, cover, and let rise in a warm spot for 1 hour or until doubled.

Make the filling: While the dough rises, cook the rhubarb, brown sugar, granulated sugar, cornstarch, cinnamon, vanilla, and salt in a saucepan over medium heat until thick and jammy (about 10–12 minutes). Let cool.

Roll and fill: Roll out the dough into a 12×16-inch rectangle. Spread the cooled rhubarb filling evenly over the dough. Roll up tightly from the long side and slice into 12 equal pieces.

Second rise: Place buns in a greased 9×13-inch baking dish. Cover and let rise for another 30–40 minutes until puffy.

Bake: Preheat oven to 350°F (175°C). Bake buns for 25–30 minutes until golden and cooked through.

Make the frosting: In a medium bowl, beat mascarpone and butter until smooth. Add powdered sugar and vanilla bean paste, and mix well. Add milk a little at a time until desired consistency is reached.

Frost and serve: Let buns cool slightly before spreading with the frosting. Serve warm or at room temperature.