Cook the bacon in a skillet over medium heat until crispy. Transfer to a paper towel-lined plate and set aside.
Mix Dijon mustard and mayonnaise in a small bowl.
Butter one side of each bread slice and dust lightly with garlic powder.
Spread the mustard-mayo mix on the unbuttered side of two bread slices.
Layer turkey, bacon, and cheddar cheese over the mustard-mayo side. Top with remaining bread, buttered side out.
Grill sandwiches in a skillet over medium heat for 3 to 4 minutes per side, pressing gently, until golden and the cheese is fully melted.
Rest for one minute, slice in half, and serve immediately.