In the bowl of a stand mixer, combine the active sourdough starter, water, brown sugar, bread flour, and salt. Using the dough hook attachment, knead on medium speed until the ingredients come together into a shaggy dough.

Add the softened butter and continue kneading. Increase the mixer speed to medium-high and knead for about 7 minutes, or until the dough is smooth, elastic, and soft.

If you don't have a stand mixer, mix the ingredients together in a large bowl, then transfer the dough to a clean work surface and knead by hand for about 10–12 minutes.

The finished dough should feel soft and slightly tacky but should not be excessively sticky. If the dough sticks heavily to your hands or work surface, add flour 1 tablespoon at a time until it becomes easier to handle.

Lightly grease a medium bowl with olive oil and place the dough inside. Cover with greased plastic wrap or a damp kitchen towel.

Allow the dough to rise at room temperature for about 4–6 hours, depending on the temperature of your kitchen. The dough should increase in volume by at least 50% and can rise to nearly double in size.

Because this is sourdough, fermentation time will vary depending on the strength of your starter and the temperature of your kitchen. Watch the dough rather than relying solely on the clock.

Once the dough has completed its first rise, cover the bowl tightly and transfer it to the refrigerator.

Refrigerate for 8–24 hours. The cold fermentation develops the sourdough flavor and makes the dough easier to handle and shape.

The next day, line a large baking sheet with parchment paper. Cut 8 small squares of parchment paper, one for each pretzel.

Remove the dough from the refrigerator and divide it into 8 equal portions. Each piece should weigh approximately 100–110 g, depending on the total weight of your dough.

Using a kitchen scale will help ensure all of your pretzels are the same size and bake evenly.

Work with one piece of dough at a time, keeping the remaining dough covered so it doesn't dry out.

Roll the dough into a long rope approximately 20 inches long.

Instead of leaving the rope round, gently flatten the dough along its length until it is about 2 inches wide. You want enough surface area to add the filling while still leaving enough dough around the edges to seal it securely.

Sprinkle approximately 1/8 of the cheddar and jalapeño filling down the center of the flattened dough, leaving a little room around the edges.

Bring the long edges of the dough together and pinch the seam firmly closed, completely enclosing the cheese and jalapeños inside the dough. Check the entire seam carefully and pinch any areas that aren't fully sealed.

Gently roll the stuffed dough back into a rope, being careful not to squeeze out the filling.

Form the rope into a large U shape. Cross the two ends over one another, twist them twice, then fold the twisted ends down toward the bottom of the U. Gently press the ends onto the base of the pretzel to secure the shape.

Place the finished pretzel onto one of the individual parchment squares.

Repeat with the remaining 7 pieces of dough.

Tip: If the dough becomes warm, soft, or difficult to work with, place it back in the refrigerator for 10–15 minutes. Chilled dough is much easier to roll, fill, seal, and shape.

Arrange the 8 shaped pretzels on a parchment-lined baking sheet, keeping each pretzel on its individual parchment square.

Loosely cover the pretzels with greased plastic wrap and allow them to rise for about 1-3 hours, or until they look noticeably puffier.

The exact time will depend on the temperature of your kitchen. You don't want them to become extremely puffy; they should simply look lighter and slightly expanded.

Add the water, baking soda, and brown sugar to a large, wide pot. Bring the mixture to a gentle boil over medium-high heat.

Carefully lift each pretzel using its parchment square and gently lower it into the baking soda bath. After a few seconds, carefully remove the parchment. It should slide away easily.

Boil the pretzels for 30–40 seconds per side. Work in batches of 2–3 pretzels at a time so you don't overcrowd the pot.

Use a slotted spoon or spider strainer to carefully flip the pretzels and remove them from the water. If a pretzel sinks slightly, gently nudge it and it should float back toward the surface.

Transfer the boiled pretzels to a parchment-lined baking sheet.

Whisk the egg and milk in a small bowl.

Brush each pretzel generously with the egg wash, making sure to coat the exposed dough.

Arrange several slices of fresh jalapeño over the top of each pretzel, gently pressing them into the egg wash so they stay in place during baking.

Return the pretzels to the oven and continue baking for approximately 5 minutes, or until the cheese is melted and bubbly and the pretzels are deeply golden brown.

Allow the pretzels to cool for several minutes before serving. The cheese filling will be extremely hot straight from the oven, so give them a little time to cool before biting into them.

These pretzels are best served warm while the cheddar filling is still gooey and melty.

Serve them on their own or with your favorite dipping sauce. They are especially delicious with beer cheese, queso, honey mustard, ranch, or a creamy jalapeño dipping sauce.