If Making With Strawberry Filling: In a small saucepan add strawberries, sugar, lemon juice, and salt and cook over medium heat, stirring occasionally until berries break down, mixture thickens, spatula leaves trail that does not fill in immediately and jam is reduced to about 1 cup, about 15 minutes. Let cool off heat for 10 minutes, then transfer to a food processor and purée until smooth. Transfer to a small bowl or clean glass jar and refrigerate until ready to use.

If Making With Cream Cheese Filling: In a food processor, add cream cheese, sugar, egg yolk, and vanilla extract. Process until well combined and sugar dissolves, about 1 minute, scraping down the sides of the food processor jar as needed. Transfer to a bowl and refrigerate until ready for use.

For the Posypka (Streusel topping): In a bowl, whisk flour, sugar, and cinnamon to combine. Add butter and rub between fingers until mixture resembles wet sand. Cover and refrigerate until ready to use.

For the Kolaches Dough: In a 2-cup liquid measuring cup or medium bowl, whisk together milk, melted butter, and egg yolks to combine (butter may form clumps; this is fine).

In the bowl of a stand mixer, whisk flour, sugar, yeast, and salt to combine. Fit stand mixer with dough hook, and with mixer running on low speed, slowly pour milk mixture into flour mixture, and knead on low speed until no dry flour remains, about 2 minutes. Increase speed to medium and knead until dough clears sides of bowl but still sticks to bottom, 8 to 12 minutes.

Transfer dough to a greased bowl and cover with plastic wrap. Proof at warm room temperature (70 to 74℉) until dough it doubled in size, 60 to 90 minutes.

Once dough has risen, line rimmed baking sheet with parchment paper; set aside. Adjust oven rack to middle position and preheat oven to 350℉ (175℃). Punch down dough and transfer to a lightly floured work surface. Roll dough into a 3-inch wide log, then cut and portion dough into twelve 2-ounce pieces.

Form each dough piece into a rough ball by pinching and pulling dough edges under so that top is smooth. On clean counter, cup each ball with your palm and roll into smooth, tight ball.

Arrange dough balls on prepared parchment-lined baking sheet, spaced evenly apart in 3 rows of 4. Brush tops with remaining 1 tablespoon melted butter. Cover loosely with plastic wrap and let proof at room temperature until puffed up but not quite doubled in size, 30 to 60 minutes.

Grease the bottom of a glass jar, cup, or a measuring cup with 2 ½- to 3-inch diameter with cooking spray or neutral oil, and make a deep indentation in center of each dough ball by slowly pressing cup into the dough on sheet. The indentations should be deep enough to fit about 1 tablespoon of filling.

Brush prepared egg wash evenly over the dough. Divide your choice of either the strawberry or cheese filling evenly among kolaches (about 1 heaping tablespoon per kolache) and smooth with back of spoon. Sprinkle streusel topping evenly over kolaches.

Bake until golden brown, about 30 minutes, rotating sheet halfway through baking. The kolaches' edges will touch and fuse together when baking. Let buns cool on pan for at least 30 minutes, then carefully pull apart. Serve warm or at room temperature.