Melt butter in a saucepan over medium-low heat. Add macaroni and American cheese; stir until cheese is mostly melted, about 2 minutes. Reduce heat to low; mix in milk. Set oven rack about 6 inches from the heat source and preheat the oven on the low broil setting. Line a baking sheet with parchment paper. Spread pretzels crackers on the prepared baking sheet; top each with a slice of sausage. Spoon 1 tablespoon macaroni and cheese mixture over sausage. Sprinkle black pepper on top. Bake under the preheated broiler until American cheese and pretzels start to brown, 1 to 2 minutes.