½tspcoarse Kosher salt(if using table salt, use half the amount)
¼tspflaky finishing salt
Preheat the oven to 350 F, and line an 8-inch baking dish with parchment paper.
In a small saucepan, melt the butter over medium heat. Add the cookie butter and stir until melted. Remove from the heat.
In a bowl, mix together the melted butter, cookie butter, sugar and brown sugar. The mixture will be grainy.
Add the egg, egg yolk and vanilla, and mix until the mixture smooths out with a "ribbony" consistency (it should easily flow off the spoon like ribbons).
Separately, whisk together the flour, corn starch and salt. Stir the dry ingredients into the cookie butter mixture until moistened. Set aside to cool for 10 minutes, then stir in the chocolate chips (reserve a few tablespoons of chocolate chips to sprinkle on top).
Use your hands or a spatula to spread or pat 2/3 of the dough into the prepared pan.
Dollop the marshmallow fluff on top of the dough, then use a small offset spatula to spread it out into an even layer.
Sprinkle the marshmallow fluff with the crumbled graham crackers.
Dollop the remaining cookie dough on top of the fluff, leaving gaps for the fluff to peek through. Sprinkle with a few more chocolate chips.
Bake the s'mores bars on the center oven rack for 25 minutes. They should be golden brown on top, with the fluff starting to puff up.
Cool the bars completely before cutting into squares. To speed up the cooling process, let them cool for 1 hour at room temperature, then refrigerate for 1 - 1 1/2 hours. Sprinkle with flaky finishing salt.
Keep leftover bars in an airtight container for up to 3 days. Since marshmallow fluff can get a little weepy, it's best to keep the bars refrigerated, then let them warm up to room temperature before serving.