Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 to 10 minutes. Drain.
Melt 1/4 cup butter in a 3-quart saucepan over medium heat. Stir flour into melted butter and cook until smooth and bubbling, about 1 minute. Mix milk, cream cheese, jalapeno peppers, Dijon mustard, black pepper, and salt into flour-butter mixture; cook, stirring occasionally, until sauce is thickened, about 5 minutes. Mix macaroni and Cheddar cheese into sauce; pour into a 2-quart casserole dish.
Mix bread crumbs, 2 tablespoons butter, and parsley together in a bowl; spread over macaroni and cheese.
Bake in the preheated oven until golden brown, 15 to 20 minutes.