Preheat oven to 350 degrees F. Line a baking sheet with foil. In a small bowl combine graham cracker crumbs, melted butter, and brown sugar. Spread crumb mixture in a thin layer on prepared pan. Bake for 4 to 6 minutes or until golden. Cool completely before crumbling into small pieces.
Line 24 muffin cups with paper bake cups. Prepare cake mix according to package directions adding 1 tablespoon key lime zest to the batter. Fill prepared muffin cups two-thirds full. Bake in a 350 degree F oven for 15 to 18 minutes or until cupcakes are golden brown and spring back when lightly pressed. Cool on a wire rack for 10 to 15 minutes. Remove cupcakes from pan and cool completely before filling and frosting.
Key Lime Filling
Meanwhile, in a medium bowl beat cream cheese, sweetened condensed milk, and 4 ounces key lime juice with an electric mixer on medium speed until smooth. Beat in an additional 1 ounce lime juice if desired. Reserve 1/2 cup filling for frosting; set aside.
Key Lime Buttercream
In a large bowl beat together butter, 3 cups powdered sugar, reserved Key Lime Filling, key lime juice, vanilla, and salt on medium speed until smooth. Gradually add remaining powdered sugar, 1 cup at a time, beating well after each addition, until icing is thick and smooth.
Using a cupcake corer or a small spoon, remove a portion of the center of each cupcake. Place filling in a large piping bag and using a large round tip fill each cupcake. Using a piping bag with a large open star tip, frost each cupcake. Sprinkle cupcakes with additional key lime zest and Graham Crumble. If desired, top each cupcake with half of a key lime slice. Serve immediately or chill for up to 3 days