Switch to the paddle attachment. Add butter 1 tablespoon at a time, beating well after each addition. Beat in vanilla. Fold in the mint until combined.
Lemon cake from Grace Parisi via Food and Wine, June 2007
Fresh mint Swiss meringue buttercream adapted from Martha Stewart’s Baking Handbook
If you want a normal sized cake, double everything (cake, frosting, filling, etc). Bake the cakes in two 9” round cake pans for about 35 minutes.