In a mixing bowl, combine the cookie crumbs and melted butter. Stir until the crumbs are fully coated and the mixture resembles wet sand. Press this mixture firmly into the bottom of a greased 9x9 inch baking pan to form an even layer.
In a double boiler or microwave-safe bowl, melt 2 cups of white chocolate chips along with the sweetened condensed milk, stirring frequently until smooth and well combined. If using a microwave, do so in 30-second intervals to avoid burning the chocolate.
Remove the melted mixture from heat. Stir in the pure lemon extract, and for an extra zest, add the optional lemon zest. If you want to make the fudge visually appealing, add the yellow food coloring at this stage and mix until evenly distributed.
Gently fold in the marshmallow fluff until fully incorporated, creating a creamy and fluffy texture.
Pour the fudge mixture over the prepared cookie crumb base in the baking pan. Use a spatula to spread it evenly and smooth the top.
Sprinkle the remaining 2 cups of white chocolate chips on top, pressing them gently into the fudge. Refrigerate for at least 3-4 hours or until completely set.
Once set, remove the fudge from the pan and cut it into squares. Serve chilled or at room temperature for a delightful treat!
