If breasts are large, butterfly each breast or halve horizontally to make two even thin cutlets (about 1/2" thick). Pat dry with paper towels and season both sides lightly with 1 tsp salt and 1/2 tsp black pepper.

Set up dredging station

In one shallow bowl combine all-purpose flour and cornstarch. In a second bowl whisk the egg with buttermilk. In a third bowl mix breadcrumbs, garlic powder, onion powder, smoked paprika, 1/4 tsp salt and 1/4 tsp black pepper.

Bread the chicken

Working one piece at a time: dredge the chicken in the flour/cornstarch mixture, shaking off excess; dip into the egg-buttermilk; press firmly into the breadcrumb mixture to coat evenly. Place on a rack and let rest 5 minutes so coating adheres.

Fry the chicken

Heat 3/8"–1/2" (about 1 cup) vegetable oil in a large skillet over medium-high heat until shimmering and 350°F if using a thermometer. Add breaded chicken, do not crowd, and fry 3–4 minutes per side until golden brown and internal temperature reaches 165°F. Transfer to a wire rack over a sheet pan lined with paper towel to drain.

Make buffalo toss (optional)

If making buffalo style: in a small saucepan melt 1 tbsp unsalted butter over low heat, stir in 3 tbsp hot sauce and 1/2 tsp lemon juice. Remove from heat and toss the hot fried chicken in the sauce until evenly coated. Return to rack for 1 minute so excess drips off.

Toast buns & assemble

Butter the cut sides of the brioche buns and toast in a skillet over medium heat until golden, about 1–2 minutes. Spread about 1½ tbsp mayo on each bun bottom. Top bottom bun with lettuce leaves, 1 tomato slice (about 1/4" thick) per sandwich, place the hot crispy chicken breast on top, add a slice of cheddar, and crown with the bun top. If cheddar doesn't melt, tent sandwiches briefly or place under a broiler 30–60 seconds to melt lightly.

Seasoning / optional side

Season the finished sandwich to taste with additional salt or pepper if desired.

Prepare chosen side (fries, chips or salad) according to package or recipe instructions and serve alongside the sandwich.