In a large bowl, whisk together the sourdough starter, warm milk, sugar, and egg.
Stir in flour and salt until a rough dough forms.
Knead in softened butter until the dough is smooth and elastic (about 5 minutes in a stand mixer or 7–8 minutes by hand).
Cover and let rise until doubled, about 4–6 hours depending on room temperature.
Turn the dough out onto a lightly floured surface. Divide into 15 equal portions and shape into balls for slider buns.
Arrange in a greased 9×13 baking dish so they are touching but not tightly packed. Cover and let rise 1–2 hours until puffy.
Preheat oven to 375°F. Bake for 40–45 minutes, until golden brown on top and cooked through.
Brush hot buns with melted butter.
Cool and slice: Let the buns cool for at least 30 minutes before slicing. For pull-apart style, slice the entire sheet of rolls in half crosswise before filling.
Notes
Baker’s Notes
Use an active starter: It should be bubbly, doubled, and slightly domed before mixing.
Warm liquids help fermentation: Keep milk around 100°F—never hot.
A sticky dough is normal: Avoid adding too much extra flour; soft dough means soft buns.
Slow rise equals flavor: Rising may take 6–8 hours in a cool kitchen. Be patient.
Shape evenly: Each dough ball should weigh roughly 70–80 grams for uniform baking.
Second rise is important: Let the buns look puffy and nearly doubled before baking.
Check oven temperature: A thermometer ensures consistent results.
Brush while hot: Melted butter gives a glossy crust and soft texture.
Cool before slicing: Cutting too soon can make the buns gummy inside.
Make ahead: Shape and refrigerate overnight, then let come to room temperature before baking.
Storage: Store in an airtight container up to 3 days or freeze for a month. Rewarm before serving.