Wash and thoroughly dry the blueberries. If the berries are particularly juicy, toss them with potato starch and sugar to help thicken the filling during baking. Set aside.

In a saucepan, melt the butter with the milk over low heat, ensuring it does not reach a boil.

In a large bowl, combine the flour, sugar, and instant yeast. Gradually pour in the melted butter mixture and add the sour cream. Mix until the ingredients come together into a rough dough.

Transfer the dough onto a floured surface and knead for approximately 5-7 minutes or until it becomes smooth and elastic.

Place the dough in a large, greased bowl, cover with a clean kitchen towel, and let it rise for about 2 hours, or until it doubles in size.

In a medium bowl, combine the softened butter, flour, and sugar. Mix with your fingers or a fork until the mixture resembles coarse crumbs. Set aside.

Once the dough has doubled in size, gently punch it down to release excess air.

Divide the dough into small, walnut-sized pieces. Flatten each piece into a circular shape.

Place a spoonful of blueberries in the center of each dough circle. Carefully gather the edges of the dough and seal them tightly, ensuring the filling remains enclosed.

Place each shaped bun seam-side down on a parchment-lined baking sheet. Repeat until all the dough and filling have been used.

Preheat the oven to 390°F (200°C).

In a small bowl, whisk together the egg and milk to create the egg wash. Brush the top of each bun with the mixture.

Generously sprinkle the prepared crumble topping over each bun.

Transfer the baking sheet to the preheated oven.

Bake for 20-25 minutes or until the buns are golden brown and the crumble is lightly crisped.

Remove from the oven and allow the buns to cool slightly before serving.