Mix together milk and rum in small bowl.
In large bowl, or mixer with paddle attachment, mix together butter, powdered sugar, vanilla and cocoa powder.
Open package of graham crackers. Using pastry brush or spoon, baste a cracker with milk/rum.
Apply thin layer of frosting to basted cracker. Baste another cracker, frost, and stack on top of first cracker. Repeat until finished.
Put cake in refrigerator, and let chill for at least an hour. Use remaining frosting on outside of cake, or choose to leave sides bare.
Top with flake salt if desired, or anything else that sounds delicious to you. Finished cake should be thoroughly chilled before slicing. Enjoy!