In a big skillet, melt the butter over medium-high heat. Whisk the flour in gradually until the mixture is smooth. Cook and whisk continually for 2 minutes. Add in salt, pepper, cayenne, and garlic; stir to combine. Gradually whisk in half-and-half and milk. Cook and whisk continually for 8-10 minutes or until mixture is thickened. Add in half of the sharp cheddar cheese; stir. Add in all of the extra-sharp cheddar cheese; stir until smooth. Take skillet off stove burner. Mix together the cooked macaroni and cheese mixture; transfer to a greased 13x9 inch casserole dish. Sprinkle with the remaining sharp cheddar cheese. Bake in a 350° oven for 20 minutes (may need to bake longer for a crispy top).