It is a pretty phenomenal moment in both book and film.  I think that you want to reference Chapter 26 in Stockett's work.  The scene is retold from Minny's point of view to Miss Celia and is referred to as "the Terrible Awful Incident."  The scene is told as Minny stands and recreates it.  The entire nature of it is told in terms of where Minny was in her relationship to Miss Hilly, the desperation she felt about her condition of being, as well as how much suffering Minny endured at the hands of Miss Hilly.  It is at this moment when Minny makes it clear that she is going to take the stand that will compel Miss Hilly to “Eat my shit."  It is at this point in which Minny realizes that revenge is the only way that she can find a point of power in her relationship to the powerful and brutal Miss Hilly.  Minny makes the pie, delivers it to Miss Hilly, who sees it as a peace offering.  In the end, Miss Hilly becomes "the woman who “ate two slices of Minny’s shit.”   It is for this reason that the secret about the pie is something that is only intimated and something that Miss Celia comes to find out at the disaster at the Benefit, and is something that she must find out from Minny herself.