Whisk flour, baking powder, and salt in medium bowl to blend. Using electric mixer, beat butter and both sugars in large bowl until light and fluffy, about 5 minutes. Add both eggs and the yolk one at a time, beating after each addition until well blended. Add bourbon-vanilla mixture. Add ½ the flour mixture, beating until just incorporated, then add the buttermilk, beat until just incorporated, following with the remaining flour mixture and, again, beating until just incorporated.