Beat butter with an electric mixer until creamy. Gradually add granulated sugar, beating until smooth. Add egg and vanilla, beating until smooth.
Combine flour, baking powder, and salt in a separate bowl; add to butter mixture, beat until just blended. Divide dough into 4 portions. Wrap each portion in plastic wrap, and chill for 1 hour plus.
With floured rolling pin; roll each portion to 1/8" thick. Using a heart shaped cookie cutter; cut cookies out and place 1" apart on greased baking sheet. Bake at 375° for 7 minutes or until edges lightly golden. Cool for 4 minutes on cookie sheets; remove to wire racks, and cool completely. Top cookies with a dollop of Royal Frosting.
Combine all ingredients in a large bowl; beat at low speed until well blended.