For the rhubarb buns, heat butter and milk in a saucepan. Mix flour, 55 g sugar, yeast, lemon and orange zest in a mixing bowl.
Add lukewarm milk and egg and knead with the dough hook of the hand mixer until a shiny dough is formed. Let this rise in a warm place for about 1 hour.
Wash and peel the rhubarb stalks and cut them into small cubes. Mix the rhubarb with 55 g sugar, lemon juice and cornstarch.
Roll out the dough to a rectangle (about 25×50) and spread the rhubarb mixture on the dough.
Roll up the dough starting from the long side and cut into 6 pieces. Place the pieces on a baking sheet lined with parchment paper, close together.
Cover and let rise again (about 30 minutes). Bake in the oven on the middle shelf at approx. 180 °C for 25-30 minutes until golden brown.
Preparation Tip:
The rhubarb buns are super to prepare and can be taken on a picnic, for example.