Ummm...Apple Walnut butter tarts are NOT butter tarts IMO.
I am a non-raisin butter tart girl myself. Why bog down that buttery, sugary goodness with icky brown blobs?
This is my mom's recipe for the filling and I think it turns out a decent butter tart. If she was in a rush, she would often just use the frozen tart shells. I omit the raisins (obviously), but will sometimes put toasted pecans in (turning them into a type of pecan pie, I guess). My mother in law adds a bit of lemon juice to hers. I'm not entirely sure why, but you could go ahead and add a 1/2 tsp. or so if you like.
Set oven to 375 degrees F. Line muffin pans with pastry. (The pastry shrinks as it bakes, so make it higher in the pan than you think.) Pour ½ cup boiling water over 1/3 cup raisins. Let stand 10 minutes then drain. Mix together 3 Tbsp. butter, ¼ cup brown sugar, ¼ cup corn syrup, 1 egg, ½ tsp. vanilla, a dash of salt and the raisins. Spoon into unbaked shells. Bake 15 to 20 minutes at 375 degrees F.