Step 1: Preheat your oven to 350°F (175°C).
Step 2: Grease a 9x13-inch baking pan lightly or line it with parchment paper.
Step 3: In a large mixing bowl, combine the yellow cake mix, softened butter, and egg. Stir until well combined.
Step 4: Crush the graham crackers into fine crumbs and fold them into the batter.
Step 5: Pour the batter into the prepared pan and spread it evenly.
Step 6: Sprinkle the white chocolate chips and mini marshmallows over the batter.
Step 7: Drizzle the sweetened condensed milk over the top.
Step 8: Bake for about 25-30 minutes until golden brown and set.
Step 9: Allow to cool completely before cutting into squares.