In a stand mixer bowl, combine flour, sourdough starter, milk, sugar, egg, and salt. Mix on low speed until the dough comes together. Add the butter, a little at a time, mixing until fully incorporated before adding more.

Once all butter is added, knead on medium-low speed for about 20 minutes, until the dough is smooth, elastic, and passes the windowpane test. Let the dough rest for 30 minutes. Then, do 2 to 3 sets of stretch and folds, spaced 30 minutes apart. Cover the dough and let it rise at room temperature until it increases by about 50%. This takes 6 to 8 hours, depending on the temperature in your kitchen. You can refrigerate the dough for up to 24 hours if you need to save time.

In a bowl, whisk together mascarpone, egg, sugar, and vanilla extract until smooth and creamy. Wet the bottom of a small glass or jar. Press a well into the center of each bun. Spoon a generous amount of mascarpone filling into each well.

Add a small spoonful of strawberry jam. Don’t overfill, as the filling will spread and rise during baking. Preheat your oven to 400°F (200°C). Bake the kolaches for 15 to 20 minutes, or until golden and the filling is set.

Let the kolaches cool slightly, then dust with powdered sugar if desired. Enjoy warm or at room temperature, with a cup of coffee or tea.