Step 1Coat 2 quart casserole with nonstick spray. Preheat the oven to 350°F.Step 2Melt butter in medium saucepan. stir in flour, mustard, paprika, and salt. slowly pour in milk, whisking continuously until there are no lumps in the flour. continue to stir with whisk, raise the heat, and cook until milk bubbles and thickens. Remove from stovetop and stir in sour cream, egg, and cheese.Step 3Toss macaroni and the cheese mixture together in a bowl and then pour it into the casserole. Top with the cornflake crumbs. Set in the oven and bake for 20 min, or until bubbling around edges.