PREHEAT oven to 350°F. Melt butter in medium saucepan on medium heat; stir in flour. Cook 1 min. Gradually add milk, stirring until well blended. Cook and stir until mixture comes to boil. Remove from heat. Add cheese, stirring until melted. Stir in mustard and bacon. Cool slightly. Stir in egg yolks, 1 at a time.
BEAT egg whites with electric mixer on medium speed until stiff but not dry, about 5 min. Gently fold into yolk mixture. Pour into 12 lightly greased nonstick muffin cups.
BAKE 25 min. or until puffed and set.