preheat oven to 350 degrees.
In a medium bowl mix together butter and brown sugar until creamy... add the corn syrup, egg, lemon juice and vanilla, continue beating until mixture is smooth.
if using nuts or raisins put a spoonful into each tart shell and then spoon the filling mixture into the shells dividing it evenly (a heaping teaspoonful should suffice)
Bake at lowest rack of oven x 15-20 minutes or until filling is slightly browned and puffy...
let cool completely before feasting!