In heavy saucepan, combine the raisins, butter, sugar, syrup and salt. Heat on a warm stove at low heat until butter is melted and the mixture is warm. Remove from heat and add vanilla and eggs. Spoon the filling into the tart shells.
Bake for 10 minutes. Reduce heat to 350ºF and bake for 5 minutes or longer until pastry is golden. Do not let the pastry filling bubble. Let butter tarts cool completely before removing from the tart tin.
Do not fill the tart shells to the top with the filling or they will spill over while baking. 2/3 full should be fine.