In A Bowl, Combine Corn Bread Mix, Egg And Milk Until Blended. Spread Into A Greased 8-In. Square Baking Pan. Bake At 400 Degrees F For 18-20 Minutes. Meanwhile, In A Saucepan, Melt Butter; Stir In Flour, Mustard And Salt Until Smooth. Add Milk. Bring To A Boil; Boil And Stir For 2 Minutes. Stir In Cheese Until Melted. Add Ham And Heat Through. Cut Corn Bread Into Squares; Top With Creamed Ham.