Meanwhile, prepare the filling. With an electric mixer, blend the cream cheese on medium-high speed until smooth. Next, add the sour cream and blend again. Add in the sweetened condensed milk, lemon zest and 2 tablespoons lemon juice, vanilla, eggs and baking cocoa. Blend again until smooth, scraping down the sides if necessary. Last, add the mashed potatoes and gently blend just until all the ingredients are sufficiently mixed.