In large bowl, beat butter with electric mixer fitted with whisk attachment until smooth. Add lemon juice and zest, vanilla and salt; blend well. Sift sugar and cornstarch together; gradually add to butter mixture, mixing well after each addition. Add milk and, if desired, icing color. Mix on high speed until fluffy. If desired, refrigerate 30 minutes before filling cooled cakes; stir before using.
Makes about 1-1/4 cups of filling, enough to fill 12 whoopie pies.