Butterscotch Filling

Whisk together brown sugar, flour, and salt in a heavy saucepan; whisk in egg yolks and milk until well blended. Cook over medium-low heat, whisking constantly, 10 to 12 minutes or until a chilled pudding-like thickness. (Mixture will just begin to bubble and will be thick enough to hold soft peaks when whisk is lifted.) Remove from heat; stir in vanilla. Use immediately.