If using grilled chicken: butterfly or pound the chicken breast to an even 1/2-inch thickness. Season both sides with 1/2 tsp salt, 1/4 tsp black pepper, 1/4 tsp garlic powder, and 1/4 tsp paprika. Marinate 15–30 minutes in buttermilk (0.5 cup) mixed with the egg for a tender result. If using fried chicken: cut or pound the breast to 1/2-inch thickness, then season the chicken the same way and let sit in buttermilk and egg for at least 20 minutes (or up to 2 hours) in the refrigerator.

Breading (for fried chicken)

Place the flour (0.5 cup) in a shallow dish and season with remaining 1/2 tsp salt, 1/4 tsp black pepper, 1/4 tsp garlic powder, and 1/4 tsp paprika. In another shallow dish combine panko breadcrumbs (0.5 cup) with a pinch of salt and pepper. Remove the chicken from the buttermilk/egg mixture, shake off excess, dredge in seasoned flour, dip back into the buttermilk/egg quickly, then press into panko to coat evenly.

Cook chicken — fried option

Heat a skillet over medium-high and add 1.5–2 tbs oil until shimmering (or heat oil to 350°F / 175°C in a deep pan). Place the breaded chicken in the pan and cook 3–4 minutes per side until golden brown and internal temperature reaches 165°F (74°C). Transfer to a rack or paper towels to rest and keep warm.

Cook chicken — grilled option

Preheat a grill or grill pan over medium-high. Lightly oil the grill surface or brush the chicken with 1 tsp oil. Grill chicken 4–6 minutes per side (depending on thickness) until nicely charred and internal temperature reaches 165°F (74°C). Let rest 3–5 minutes.

Cook bacon

While the chicken rests, cook bacon in a skillet over medium heat 3–5 minutes per side until crisp. Transfer to a paper towel-lined plate to drain. For faster prep, bake bacon on a sheet at 400°F (200°C) for 12–15 minutes.

Toast buns and assemble sauce

Butter the cut sides of the bun (1 tsp butter) and toast in a skillet or on the grill until golden, about 1–2 minutes. Mix mayonnaise (1 tbs) with mustard (1/2 tsp) in a small bowl and spread on both bun halves.

Assemble sandwich

Place the bottom bun on a plate. Add lettuce leaf, then the cooked chicken (choose fried or grilled), top with cheddar slice so it melts slightly from the warm chicken, then add tomato slices and two strips of bacon. Spread any remaining sauce on the top bun and close the sandwich. Cut in half if desired and serve immediately.

Serve

Serve hot with fries, chips, or a side salad. Leftover chicken can be refrigerated up to 3 days.