Heat the oven to 425°F/220°C, or 200°C for a fan oven. Let the oven fully preheat so the chicken starts roasting immediately instead of slowly drying out.
Place each chicken breast between parchment or plastic wrap and gently pound the thickest area to 1.5cm / 0.6″. The thick end should closely match the tapered end so the whole breast reaches doneness together.
Stir all of the seasoning ingredients together in a small bowl. Break up any brown sugar clumps so the rub coats the chicken evenly.
Cover a baking sheet with foil, then top it with baking parchment paper and place the chicken smooth-side down. Drizzle about 1 tsp oil over the chicken, rub it in, and scatter seasoning over the surface.
Turn the chicken over and coat the second side with the remaining 1 tsp oil and seasoning. Press the rub lightly onto bare spots so both sides develop golden spice edges.
Roast for 18 minutes, checking the thickest center with a meat thermometer. The chicken is ready when the surface is golden and the center reaches 165°F/75°C.
Move the chicken from the baking sheet directly to serving plates. Getting it off the hot pan prevents the underside from continuing to cook too aggressively.
Rest the chicken for 3, 5 minutes, then add finely chopped parsley if desired. The meat should feel lightly firm and release less juice when sliced after resting.