Cream together the butter, margarine, sugar and vanilla.
Sift together the flour, bicarb soda and salt. Add to sugar mixture and mix well, being careful not to over mix.
Drop heaping teaspoonfuls of dough on ungreased baking trays and flatten each slightly.
Bake for 13 to 15 minutes. After each tray is removed from the oven, carefully place 5 or 6 white chocolate chips on top of each biscuit while they are still hot. When chocolate melts, spread with a knife.