10:00 PM: In a small bowl, whisk your 10g of starter culture into 30g of water. Add 60g of bread flour and mix thoroughly. Knead briefly with your hands to form a tight, smooth ball, then roll it into a firm cylinder.

Place the cylinder into a tall glass jar and fill it completely with room-temperature water 74–78°F (23–26°C) so the dough is fully covered. Let it sit out overnight for 8 to 10 hours. As it ferments, it will expand, trap gas, and float proudly to the top of the water.

8:00 AM: Carefully lift the floating starter out of the water jar. Discard the wet, melted outer surface and scale out exactly 60g from the firm core center to use in your dough.

In your stand mixer bowl, combine the 60g of extracted stiff starter, 150g of milk, 50g of eggs, 300g of bread flour, and 75g of sugar (add the optional 0.5g of dry yeast here if using). Mix briefly until a shaggy mass builds, cover, and let autolyse for 30 minutes.

8:30 AM: Attach your dough hook. Mix the dough on low speed for 2 to 3 minutes (or if using a KitchenAid mixer, run it on speed 3 for 3 to 4 minutes) until well incorporated.

Add the 6g of salt and continue mixing for 2 minutes until a clean ball takes shape.

Increase your mixer speed to medium-high and begin introducing the 60g of soft butter a tablespoon at a time. Knead intensely for 10 to 15 minutes until the dough builds enough gluten strength to clear the sides of the bowl. Note: If it struggles to bundle together, feel free to add a little more flour (20g to 40g).

9:00 AM: Cover the bowl tightly and let it proof for 3 to 4 hours at a warm temperature of 76–80°F (24–28°C). Perform 2 separate rounds of structural stretches and folds during this window; the dough will become light and slightly puffy.

12:00 PM: Transfer the covered dough container directly into the refrigerator for a long cold fermentation rest until the next morning.

7:00 AM: Turn the cold dough out onto a workspace and divide it cleanly into 10 uniform pieces weighing exactly 60g each. Round each piece into a tight ball, cover, and let them rest for 15 minutes to relax.

Prepare the Filling: Cut your strawberries and rhubarb into small pieces. In a small bowl, thoroughly combine the 6 to 8 tablespoons of sugar with the 1 tablespoon of rice starch.

Roll each dough ball out into a thin circle. Place a generous spoonful of fruit right in the middle, then top it with 1 teaspoon of your sugar-starch mixture.

Gather the opposite sides of the dough circle up to the middle and pinch the edges together firmly to form a tight, secure seal.

Arrange each piroshok seal-side down on a tray lined with parchment paper, leaving about 1 inch of distance between them. Cover loosely and let them proof on the counter for 2 to 4 hours until they look visibly bigger and puffier.

Preheat your oven to 375°F (190°C). Gently coat the tops of the puffed rolls with a quick egg wash.

Slide the tray into the oven and bake at 375°F (190°C) for exactly 10 minutes to set the oven spring. Lower the temperature down to 350°F (175°C) and continue baking for an additional 10 to 15 minutes until beautifully golden brown.

The Sweet Glaze: While they bake, mix your 6 tablespoons of warm water with 3 tablespoons of sugar until completely dissolved. As soon as the piroshki come out of the oven, brush this sweet syrup evenly over the hot tops. Let them cool completely before serving.