In a large bowl, whisk together 2 cups all-purpose flour and 1/2 tsp salt. Add the 6 tbs cold cubed butter. Using a pastry cutter or your fingertips, work the butter into the flour until the mixture resembles coarse crumbs with pea-sized butter pieces remaining.

Bring dough together

Combine 1/2 cup cold water with 1 tsp white vinegar. Drizzle the liquid into the flour-butter mixture, a little at a time, stirring with a fork until dough just comes together. Add a splash more water if needed—dough should be cohesive but not sticky.

Rest dough

Gather dough into a disk, wrap in plastic, and chill in the refrigerator for at least 30 minutes while you prepare the filling. Chilling relaxes the gluten and firms the butter for a flakier fried pastry.

Filling

Prepare jalapeños

While dough chills, rinse and dry 4 jalapeños. Slice in half lengthwise, remove seeds and membranes if you want milder heat (reserve seeds if you prefer hotter). Finely dice the jalapeño flesh. For milder pastry, use 2 jalapeños and 2 poblano or green bell pepper halves instead.

Mix filling

In a medium bowl, combine 1.5 cups finely grated cheddar, 3 tbs softened cream cheese, 2 tbs finely chopped cilantro, 1/2 tsp garlic powder, 1/4 tsp black pepper and 1/4 tsp salt. Fold in the diced jalapeños until evenly distributed. Taste a small amount and adjust seasoning if needed.

Assembly & Frying

Roll out dough

Remove the dough disk from the refrigerator and let rest 5 minutes if very firm. Lightly flour a work surface with 1–2 tbs flour. Roll the dough to about 1/8-inch (3 mm) thickness. Using a 3.5–4 inch round cutter (or a knife), cut out circles. Gather scraps, reroll once and cut more rounds. You should get about 12–14 rounds.

Fill and seal

Place about 1 to 1.5 tablespoons of the jalapeño-cheese filling in the center of each round. Brush the edges lightly with the egg wash (1 beaten egg + 1 tbs water). Fold the dough over to form a half-moon and press edges together to seal. Crimp edges with a fork to ensure a tight seal and prevent leakage. Lightly dust any very wet edges with a bit of flour if needed.

Heat oil

Pour about 3 cups vegetable oil into a deep, heavy skillet or pot so it is 1.5–2 inches deep. Heat the oil over medium-high heat until it reaches 350°F (175°C) — if you don't have a thermometer, test with a small scrap of dough: it should sizzle and brown gently in ~45–60 seconds.

Fry pastries

Working in batches of 3–4 (don't overcrowd), carefully lower the sealed pastries into the hot oil. Fry, turning occasionally, until golden brown on both sides, about 2.5–3.5 minutes per batch depending on thickness. Use a slotted spoon to transfer fried pastries to a wire rack or paper towels to drain. If oil temperature drops, raise heat slightly; if too hot, reduce to avoid burning before the pastry cooks through.

Finish

Let fried pastries rest 2–3 minutes (cheese will be very hot). Optionally sprinkle with a little flaky salt or extra chopped cilantro. Serve warm with lime wedges and sour cream or crema for dipping.