Preheat the oven to 350°F. Line 2 cookie sheets with parchment paper.
In medium bowl, beat butter and sugars with electric mixer on high speed until light and fluffy. Add egg and vanilla, beating until combined. Add liqueur, beating until well combined.
In another medium bowl, combine the flour, baking soda and salt. Add to butter mixture, beating on low speed until dough forms.
Shape dough into 1-inch balls; place on the prepared baking sheets. Press a kiss into each cookie ball, letting the dough form up around the kiss. Bake for 8 to 9 minutes or until lightly golden. Cool on the cookie sheets for a few minutes; remove to a wire rack to cool completely.