Line an 8×8 inch baking pan with parchment paper and lightly grease it
In a medium bowl, mix crushed graham crackers and melted butter until the mixture resembles wet sand. Press evenly into the prepared pan
Spread sweetened condensed milk in an even layer over the crust
Melt semi-sweet and milk chocolate chips in separate microwave-safe bowls or in a double boiler
Pour melted semi-sweet chocolate over the condensed milk and use a spatula to spread it evenly
Sprinkle chopped nuts (if using) over the semi-sweet layer
Drizzle melted milk chocolate over the top and gently swirl with a toothpick or spatula
Sprinkle mini marshmallows (if using) and press lightly into the chocolate
Chill the bars in the refrigerator for at least 2 hours
Once set, lift the parchment paper to remove the bar from the pan. Cut into 16 bars and serve