1. Boil the potatoes for the dough in some salted water until done. Rinse with cold water, peel, and press through a potato ricer, or mash them with a fork. Set aside and let cool down completely.

2. For the blueberry filling, mix water, sugar, and cornstarch in a small saucepan. Add the blueberries (they can still be frozen) and bring everything to a boil, then let simmer while stirring constantly, until the liquid in the pot has thickened. Set aside and let cool down completely.

3. For the streusel topping, mix flour and sugar in a bowl. Add the soft butter and vanilla extract and rub everything between your fingers until you get different-sized streusels. Place in the fridge until needed.

4. For the dough, crumble the yeast into the lukewarm milk. Add a little of the sugar and mix until the yeast has dissolved. Let bubble up for about 10 minutes. Add flour, salt, egg, mashed potatoes, and 100g (3.5 oz.) of the butter to a large bowl. Add the yeast milk and knead everything with the dough hooks of a food processor for about 10 minutes until a smooth dough forms. Cover the bowl and let the dough rise in a warm place for about 1 hour.

5. Divide the risen dough into two pieces and roll out each on a floured surface to get two circles with a diameter of about 30cm each. Brush one of the dough circles with the remaining butter, place the other dough circle on top, and roll out both circles (together) to create an even larger dough circle with a diameter of about 40cm. Cut into 12 slices (like a pizza) and add some blueberry filling on the wide end of each slice. Roll the croissants up from that wide side and enclose the filling by doing so. Place the rolled-up croissants on two baking sheets lined with baking parchment, cover each with a kitchen towel and let the croissants rise another 20 minutes or so.

6. Preheat the oven to 180°C (350°F). Mix the egg yolk with some milk and carefully brush the croissants with it, sprinkle the streusel on top, and bake for about 20-22 minutes. Switch the baking sheets halfway throu