1 cup dark chocolate chips
½ cup semisweet chocolate chips
1 cup unsalted butter
2 egg yolks
1½ cups dark brown sugar
¾ cup granulated sugar
1 teaspoon kosher salt
2 teaspoons vanilla extract
1½ cups potato chips, crushed
1¼ cup walnuts, toasted
1 teaspoon instant espresso powder
⅓ cup unsweetened cocoa, sifted
1 cup flour
1 cup marshmallows, roughly chopped
Preheat oven to 350°F.
Grease and line a 9x13-inch baking dish with parchment paper. Set aside.
In a small pot, melt butter completely, then pour over chocolate chips in a medium-sized bowl. Allow to sit for 3 minutes, then whisk together until smooth.
Place brown sugar, granulated sugar, kosher salt, vanilla, and eggs in a large bowl. Use a hand mixer to whip ingredients, increasing speed slowly from low to medium high. Continue whipping for 3 to 5 minutes, or until the mixture is very light and pale in color.
Pour the melted butter and chocolate into the bowl and mix for 2 to 3 minutes, until fully combined.
Add walnuts and 1 cup of the potato chips to the batter, and fold with a spatula to incorporate. Add cocoa, espresso powder, and flour, then slowly fold until no flour streaks remain.
Spread the batter in the prepared pan and top with remaining ½ cup of potato chips. Place in the oven and bake for 30 minutes.
Carefully remove the pan from the oven and press the marshmallows into the batter. Place back in the oven for 3 to 5 minutes, or until the marshmallows are toasted.
Cool completely, then slice into 15 squares.