- Heat the milk to 104F (40C), add sugar, and stir until dissolved. Add the yeast, and leave for 20 minutes covered with a towel until a fluffy cap is formed.
- Put the egg, salt, vanilla extract and butter at room temperature into a mixer bowl. Knead everything with a whisk, add the leaven, change the whisk to the hook and gradually add the flour. Knead for about 10 minutes.
- Let the dough rise in a warm place for at least 1 hour, covered with a tea towel, until doubled in size.
- Put the frozen berries in a saucepan, add sugar and starch through a strainer. Simmer over a medium heat, stirring until the mixture thickens and boils.
- Divide the risen dough into 2 balls. Roll out each part into a rectangular layer on a table powdered with flour.
- Grease the dough with berry filling.
- Roll the long side of the rectangle into a roll and cut into pieces.
- Spread the buns in a mold at a distance of about 2cm (0.75 inch) from each other, cover with a towel, and leave for at least 30 minutes until they increase in volume.
- Put the buns in the oven preheated to 356F (180C) for 25 minutes.
- For the glaze, whip all the ingredients until creamy, and grease the warm buns.