In a separate bowl, sift the flour, baking powder and salt. Stir the milk and vanilla together. Add the flour and the milk alternately to the butter mixture, beating well after each addition. Spoon the batter (using an ice cream scoop is best) into the cupcake cups so they are two thirds full and bake for about 18 minutes, until a toothpick inserted comes out clean. Cool the cupcakes in the tins.