For the crust: Preheat the oven to 350 degrees F. Spray an 8-by-8-inch pan with cooking spray, or line with foil and spray with cooking spray for easy cleanup. In a medium bowl, mix the cookie crumbs and melted butter. Press into the prepared pan. Bake for 10 minutes. Let cool while you prepare the filling.
For the fudge: Place the white chocolate chips in a medium saucepan. Add the sweetened condensed milk and warm over medium-low heat, stirring, until melted.
Once the mixture in the pan has melted, remove it from the heat and stir in the lemon extract, lemon zest, and food coloring if using. Pour two-thirds of the fudge over the crust and spread. Working quickly, add the marshmallow cream to the remaining fudge in the saucepan. Fold, being careful to not totally mix them together; you want swirls of fudge and marshmallow. Pour and spread over the fudge already in the pan.
Let cool to room temperature and then refrigerate to set. Slice into squares or triangles and serve. Cover and store at room temperature or in the refrigerator. The fudge will for keep 3 to 4 days.
"Dessert Mashups: Tasty Two-in-One Treats Including Sconuts, S'morescake, Chocolate Chip Cookie Pie and Many More" by Dorothy Kern © Ulysses Press 2014. Reprinted with permission from Ulysses Press. All rights reserved.