Preheat the oven to 350F. Line an 8 by 8-inch baking dish with 2 pieces of parchment paper so it hangs over all 4 sides.
Whisk together the butter, white sugar, and brown sugar.
Whisk in the egg, and then the vanilla.
Add the flour and salt to the butter mixture, and whisk just to combine, being careful not to over-mix.
Stir in the chocolate chips, mini marshmallows, and crushed graham crackers.
Pour the batter into the prepared dish. Bake until the blondies are golden and set along the outside, but still a touch doughy in the center, about 25 minutes.
Cool completely before cutting.
Notes
Store blondies in an airtight container at room temperature for up to 1 week.