Lay bacon in a cold skillet. Cook over medium heat until crisp, 7–9 minutes.
Transfer bacon to paper towels. Reserve about 1 teaspoon drippings in the skillet.
Stir butter, mayonnaise, and garlic powder in a small bowl until smooth.
Spread the mixture on one side of each bread slice.
Flip two slices, then layer cheddar, bacon, and black pepper. Top with remaining slices, buttered side out.
Warm skillet over medium-low. Cook sandwiches covered until golden and melty, 3–4 minutes per side.
Rest 1 minute, slice, and serve hot.