In a large mixing bowl, combine warm milk, active dry yeast, and a pinch of sugar. Allow it to sit for about 5-10 minutes until bubbly.

Add in the all-purpose flour, eggs, and softened butter, then knead the mixture for about 10 minutes until smooth and elastic.

Place the dough in a greased bowl, cover it with a damp cloth, and let it rise in a warm spot until doubled in size, approximately 1-1.5 hours.

While the dough rises, wash and chop fresh rhubarb into small pieces. In a saucepan over medium heat, combine the rhubarb, granulated sugar, and lemon juice. Cook for about 15-20 minutes, stirring occasionally, until the mixture thickens and the rhubarb breaks down.

Remove from heat and let the jam cool to room temperature.

Once the brioche dough has risen, punch it down to release air. On a floured surface, roll out the dough into a large rectangle about ¼ inch thick. Spread a generous layer of the cooled rhubarb jam evenly over the surface.

Carefully roll the dough tightly from one end to the other, then slice it into equal pieces, about 1-2 inches thick. Place the cut buns in a greased baking dish.

Cover the baking dish with a clean kitchen towel and allow the rolled buns to proof for about 30-45 minutes in a warm area.

Preheat your oven to 350°F (175°C). Once the buns have proofed, bake for 25-30 minutes until golden brown.

While the buns are baking, prepare the Vanilla Bean Mascarpone Frosting. In a mixing bowl, combine mascarpone cheese, powdered sugar, and vanilla bean paste. Whip until smooth.

If the frosting is too thick, add a splash of heavy cream to reach a spreadable consistency.

Once your buns are out of the oven, let them cool for a few minutes before generously spreading the frosting over them.