melted butter in the filling???

I see you're using the "improved" version, which focuses on the crust. It may also be helpful for you to look at the original, http://food52.com/recipes/1374-lazy-mary-s-lemon-tart which does a more thorough job of describing how you just throw everything into a food processor, lemon peel and all.

Whoops, bad memory of the wording, sorry, but no, you don't melt the butter for the filling. Soft or chilled should be fine.

Chris, I believe that instruction refers to prep off the crust, not the filling. I am asking about the butter used in the filling; melted, softened, chilled???

No. Notice where she says "don't bother to fully break down the butter." You'll love this recipe.