Preheat the oven to 350°F.

Spray or lightly butter the insides of a square 8×8 inch baking dish and line it with parchment paper, leaving a few inches of parchment overhang to help you remove the bars later.

Melt the butter in a pot over medium heat, stirring constantly. Reduce heat to medium-low when the butter starts to bubble, then cook for another 5-10 minutes until it’s browned and smells nutty but not burnt. Place the pot with the melted in a bowl of ice and whisk it to cool the butter so that it’s soft (not hard) but no longer melted.

Mix the dry ingredients (flour, baking soda, salt) together.

In a large mixing bowl, mix together the softened brown butter and the sugars with a spoon/spatula or electric hand mixer until the texture resembles wet sand. Mix in the eggs, one at a time, then the vanilla. Be careful not to whip too much air into the mixture.

Add the dry ingredients to the bowl with the wet ingredients and mix until just combined.

Using a spoon/spatula, mix in the graham crackers, ½ cup mini marshmallows, and ½ cup chopped chocolate bars. Be careful not to overmix.

Press the dough into the prepared baking dish in an even layer, making sure it gets to the edges. Bake for 25-30 minutes, until the cookie base is cooked through.

Remove cookie bars from the oven and top them with ½ cup chocolate squares and 1 cup marshmallows, then place them back in the oven. Broil for 1-2 minutes (rotate pan halfway through), depending on how toasted you want your marshmallows.

Let cookie bars rest for 30 minutes in the pan, then remove them from the pan and place them on a rack until firm and completely cooled. Once cooled, cut them into 16 squares. Note: cutting them too early may make them fall apart.