The sweet levain is optional, but using a regular sourdough starter will give the buns a tangy flavor. The night before you plan to make the dough, add the sourdough starter to a clean jar. Stir in the sugar and water until it looks milky. Then, mix in the flour until there are no clumps. Cover the jar loosely and let it sit overnight for 10-12 hours. Ensure the sweet levain has doubled in size and is bubbly before using.

You can mix this recipe by hand or with a stand mixer. Warm the milk in the microwave for about 30 seconds and pour it into the mixing bowl. Add the water, sweet levain, and sugar, stirring until most of the levain has dissolved. Whisk in the egg until everything is combined.

Next, add the flour, salt, and room temperature butter (cut into cubes) and mix until a shaggy dough forms. Cover the dough with plastic wrap and let it rest for 30 minutes.

After resting, knead the dough for 5-7 minutes until it is smooth and not sticky, using flour as needed to prevent sticking. Once kneaded, place the dough in a greased bowl (with oil or butter) and let it rest for another 30 minutes.

After this second rest, perform one stretch and fold to the dough to promote gluten development. Allow the dough to rise until it has doubled in size, which will take about 4-6 hours depending on your kitchen temperature. If you need more time, you can refrigerate the dough overnight after the first rise and shape it the next day.

Once the dough has doubled, turn it out onto a lightly floured surface and divide it into 10 equal pieces (about 80 grams each). Roll each piece into a rectangle measuring 5x8 inches, then roll it into a log that is 5-6 inches long, ensuring to tighten the seams as you roll.

Place each shaped bun on a hot dog bun pan or a parchment-lined baking sheet, spacing them about an inch apart. Cover the pan with oiled plastic wrap and let the buns rest until they have doubled in size, which should take about 2-3 hours. They should feel puffy and light to the touch; do not rush this step to avoid dense buns.

If needed, the shaped buns can be refrigerated overnight and baked the next day. When ready to bake, preheat the oven to 375 degrees F. Mix the egg and water together and brush the mixture over the buns. Bake for 20-25 minutes.

The sourdough hot dog buns are done when they reach an internal temperature of 190-200 degrees F. For added softness, brush the buns with melted butter right after taking them out of the oven. Allow the buns to cool for 20-30 minutes before enjoying them warm.

To store, keep the cooled buns in an airtight container or zip-top bag at room temperature for up to 3 days. For longer storage, freeze the buns for up to 2 months and thaw them at room temperature before serving. Warm them slightly for the best soft and fluffy texture.