Preheat oven to 220 °C. Make sure the rack is in the centre of the oven. Sift flour, baking powder, salt and sugar together. Beat eggs, vanilla and milk together. Add melted butter. Combine egg and milk mixture with sifted ingredients. Mix through peanut butter. Mix through, but not too much. Spoon into greased muffin tins (three quarters full.) Spoon another teaspoonful of peanut butter on top of every muffin and bake for 15 – 20 minutes.