Season both sides of chicken breasts with garlic powder, smoked paprika, cumin, salt, and pepper. Let rest for 10 minutes.
Heat olive oil in a large skillet over medium-high heat. Cook chicken for 6-7 minutes per side until internal temperature reaches 165°F. Remove and rest for 5 minutes before slicing thinly.
Mix mayonnaise and Dijon mustard in a small bowl. Spread on one side of each bread slice.
On two bread slices, layer cheese, sliced chicken, avocado slices, and another slice of cheese. Top with remaining bread, spread side facing out.
Butter a skillet over medium heat. Grill sandwiches for 3-4 minutes per side until golden and cheese is melted. Press gently with spatula. Cool 2 minutes before serving.