Combine dark chocolate and butter in a medium microwave-safe bowl and melt on HIGH heat in 30 second bursts, stirring after each interval until smooth (about 1½ to 2 minutes). Allow the mixture to cool for 5 minutes.
Whisk in the granulated and brown sugar, then the eggs (one at a time), and finally the vanilla.
Fold in the flour mixture just until combined (do not overmix). Fold in the chocolate chips (if using).
Pour batter into the prepared baking pan, and smooth the top with a small offset spatula. Top with the Whiskey Marshmallows in a grid pattern (4 x 4), pressing them in gently.
Bake the brownies for 35 to 38 minutes or until a cake tester or toothpick inserted in the center comes out clean (or maybe with just a few crumbs attached). It might be hard to tell if the center of the brownies are set because of the melting marshmallows. I'd err on the side of caution and go for the longer baking time if necessary. Once done, remove the brownies from the oven and cool completely in the pan on a wire rack.
Carefully lift parchment to remove the brownie slab from the pan and place it on a cutting board. Cut it into 2-inch squares (around each marshmallow) with a sharp knife, wiping off the knife between cuts. Serve with a large glass of milk (or your favorite non-dairy alternative) and enjoy!
You can use regular unsweetened cocoa powder if you prefer. The brownies just won’t have as deep a chocolate flavor.Adding espresso powder to the brownie batter helps deepen the chocolate flavor. It’s optional, but highly recommended.While in this case the whiskey is just there for flavor, you can substitute in 2 teaspoons of pure vanilla extract when making the marshmallows if you prefer.For slightly thinner brownies, use a 9- x 9-inch inch baking pan. Reduce the baking time by about 5 minutes (but start checking at the 30 minutes mark).