Beat the butter at high speed for 3 minutes, then add the cream cheese and beat on low about a minute. Do the same if you use shortening.
Stir the salt, cream of tartar and vanilla into ¼ cup of the milk until dissolved. Add to the butter/cream cheese mixture and beat well.
Add the powdered sugar a little at a time until well mixed and smooth. Add more milk if the frosting is too thick.
Fit the large tip into a decorator bag, and fill with frosting. Swirl "roses" on top of each cupcake. Add a candy melt carrot to each, and add green sprinkles or green tinted coconut if desired, to look like leaves and greenery.