For the frosting, add the butter, lemon juice and cream cheese in a large bowl. Beat with an electric mixer on medium for 2 minutes. Turn the mixer down to low gradually add the powder sugar, vanilla and lemon zest. Once combined, turn the mixer up to medium and beat for another two minutes. Spread onto cooled cake then add chocolate chips to cake and serve. Store in refrigerator for up to 5 days. Enjoy!!