Prepare an 8x8 baking dish by spraying lightly with oil, or lining with parchment paper. Set aside. In a bowl, cream together the butter and sugar, beating for 4-5 minutes. Add the vanilla, salt and 1/2 C flour. Beat well before adding the rest of the flour. Beat until completely combined(may still be crumbly). Press into the prepared 8x8 pan and poke gently with a fork all over the dough. The dough may be crumbly, just gently press any broken pieces back into place. Bake shortbread 17-18 minutes until barely golden around the edges. Remove from the oven and allow to cool a little while preparing the filling.