Preheat oven to 400 degrees F (200 degrees C). Spray 12 muffin cups with cooking spray or grease with butter.
Heat butter in a saucepan over medium heat. Cook and stir garlic in the melted butter until fragrant, about 1 minute. Add flour to garlic mixture; cook and stir until mixture is smooth and thickened, about 2 minutes.
Slowly pour milk into flour-butter mixture while continuously stirring with a whisk until sauce is smooth and thickened, about 5 minutes. Remove saucepan from heat and stir Parmesan cheese into sauce until cheese melts from the heat of the sauce; season with salt and pepper.
Divide the potatoes among the 12 prepared muffin cups and spoon cheese sauce over potatoes.
Bake in the preheated oven until potatoes are tender, about 25 minutes.