Preheat your oven to 450°F (230°C). Line a rimmed baking sheet or shallow casserole dish with parchment paper or aluminum foil.

In a medium bowl, combine the softened cream cheese, shredded sharp cheddar, Ranch seasoning, Worcestershire sauce, garlic powder, and black pepper. Mix until smooth and well-incorporated. Set aside.

Using a sharp knife, cut a horizontal pocket into the thickest part of each chicken breast, being careful not to cut all the way through to the opposite side or bottom. Create a cavity that opens like a book.

Divide the cheese filling into four equal portions. Spoon one portion into each chicken pocket, spreading it around inside. If the opening seems loose, secure with one or two toothpicks inserted horizontally.

Brush the outside of each stuffed chicken breast with melted butter, coating all surfaces. Sprinkle both sides evenly with salt, black pepper, garlic powder, and paprika.

Arrange the stuffed chicken breasts on the prepared baking sheet, spacing them a few inches apart. Bake for 25 to 30 minutes, until the chicken reaches an internal temperature of 165°F (74°C) and the exterior is golden brown.

If using the lemon butter finish, whisk together the butter, lemon juice, and fresh thyme. Drizzle over the hot chicken immediately after removing from the oven.

Let the chicken rest for 5 minutes before serving to allow the juices to redistribute and the cheese filling to set slightly.