Place a rack in middle of oven; preheat to 350°. Generously coat a 13x9 baking dish with nonstick vegetable oil spray. Cook 4 oz. bacon (4-6 slices) in nonstick skillet over medium heat, turning occasionally, until browned and crisp, 8-10 minutes; transfer to paper towels to drain. Let sit until cool enough to handle, then finely chop.

Whisk one 8.5-oz. box corn muffin mix and 2 cups half-and-half in a medium bowl until smooth. Add 6 large eggs and 2 tsp. Diamond Crystal or 1?¼ tsp. Morton kosher salt and whisk to combine. Stir in bacon, ?½ cup coarsely grated extra-sharp cheddar, ?½ cup chopped drained pickled jalapeños, patted dry, and 2 Tbsp. finely chopped chives; scrape batter into prepared pan.

Bake casserole until slightly puffed and just set (center should still wobble slightly when casserole is gently shaken), 25-30 minutes. Let cool 10-15 minutes before cutting into squares.

Do Ahead: Casserole can be baked 5 days ahead. Let cool, cover, and chill.