Mix the Starter & Wet Ingredients: In the bowl of your stand mixer or your mixing bowl, combine the active sourdough starter and room temperature milk. Stir gently until the starter is mostly dissolved.

Add Dry Ingredients: Add the flour, sugar, and salt to the bowl. Mix on low speed with the dough hook until a shaggy dough forms, about 1-2 minutes. Or mix with a spatula.

Incorporate the Egg: Add the room-temperature egg to the dough. Mix on low speed for 2-3 minutes or mix with spatula until the egg is fully incorporated and the dough comes together. It might look a little messy at first, but keep going!

Add Butter Gradually: With the mixer running on low speed, add the softened butter cubes one at a time, waiting until each piece is almost fully incorporated before adding the next. This process might take 5-10 minutes.

Knead the Dough: Once all the butter is in, increase the mixer speed to medium-low and knead the dough for about 10 minutes. If you are kneading by hand, this will take longer (up to 20 minutes). Using a bench scrapper will make the job easier. The dough will start sticky but should become smooth, elastic, and pull away from the sides of the bowl. Perform a windowpane test: stretch a small piece of dough thinly; if you can see light through it without it tearing, your gluten is well-developed.

Bulk Fermentation: Transfer the dough to a lightly oiled large bowl. Cover tightly with plastic wrap. Let it bulk ferment at room temperature (around 70-75°F / 21-24°C) for about 12 hours (more or less depending on the room temperature), or until it has noticeably increased in volume (50% or more, but not double).

Tip: Use a straight sided container for an accurate read of the dough rise.

Cold Proof (The Magic Happens!): Place the covered bowl in the refrigerator for a minimum of 12 hours, and up to 48 hours. This slow, cold fermentation is crucial for flavor development.

Shape the Buns: The next day (or day after), lightly flour your work surface. Gently turn the cold dough out onto the surface. Divide the dough into 6 equal pieces (or 4 if you want larger burger buns).

How to Shape: Take each piece and gently pull the edges up and into the center, creating a tight ball. Pinch the seams together at the bottom. Then, with a cupped hand, gently roll the ball against the countertop to create tension and a smooth surface.

Second Proof (Final Proof): Place the shaped buns on a baking sheet lined with parchment paper, spaced a few inches apart. I like to press them down gently before letting them proof. This will help giving them that burger bun shape, instead of a ball shape. Cover loosely with plastic wrap or a damp kitchen towel. Let them proof at room temperature for 2-4 hours, or until visibly puffy and almost doubled in size. They should feel airy and jiggle slightly when gently nudged.

Preheat Oven & Egg Wash: About 30 minutes before baking, preheat your oven to 375°F (190°C). In a small bowl, whisk together the egg and water for the egg wash.

Bake: Gently brush the tops of the proofed buns with the egg wash. If desired, sprinkle with sesame or poppy seeds. Bake for 20-30 minutes at 375°F (190°C), rotating the baking sheets halfway through, until the buns are deep golden brown and sound hollow when tapped on the bottom.

Cool: Transfer the baked buns to a wire rack to cool completely before slicing and serving.