Prepare the Chicken: Place each chicken breast between two sheets of cling film or baking parchment. Using a meat mallet or a rolling pin, gently pound them until they are an even thickness of about 1.5 cm. This ensures even cooking and a tender result. In a medium bowl, combine the olive oil, smoked paprika, garlic powder, onion powder, dried thyme, black pepper, and salt. Add the pounded chicken breasts and toss them thoroughly, ensuring each piece is well coated in the vibrant, reddish-brown spice mix. Let them marinate at room temperature for at least 15 minutes, or in the refrigerator for up to an hour for deeper flavour.

Preheat the Grill: Heat a griddle pan or outdoor grill to a medium-high heat (around 200°C / 400°F). You'll know it's ready when you see a faint wisp of smoke rising from the pan or grill grates, indicating it’s hot enough to sear properly.

Grill the Chicken: Carefully place the marinated chicken breasts onto the hot grill. You should hear a satisfying sizzle as they hit the hot surface. Cook for 4-6 minutes per side, or until the chicken develops beautiful char marks and is cooked through. The chicken should turn from a pale pink to an opaque white throughout, and when pressed gently, it should feel firm. Use a meat thermometer to check for an internal temperature of 74°C (165°F) for food safety.

Melt the Cheese: Once the chicken is nearly cooked, about 1-2 minutes before removing it from the grill, place two slices of Cheddar cheese on top of each chicken breast. Cover the grill or pan with a lid (or a piece of foil if you don't have a lid) for about a minute. The cheese will soften and melt into a glossy, inviting blanket over the chicken.

Toast the Buns: While the chicken rests briefly off the heat, lightly butter the cut sides of the brioche buns. Place them cut-side down on the same hot griddle pan or grill for 1-2 minutes, or until they are golden brown and lightly toasted. You'll smell the sweet, buttery aroma of the brioche as it toasts.

Assemble the Sandwiches: Spread a generous amount of mayonnaise on both the top and bottom halves of each toasted brioche bun. On the bottom bun, layer a few crisp Cos lettuce leaves, followed by a couple of slices of fresh tomato. Carefully place a cheesy grilled chicken breast on top of the tomato and lettuce. Add a few pickle slices if using, then crown it all with the top bun. Serve immediately and enjoy the warm, yielding texture and layered flavours.