Prep: Preheat your oven to 350° F. Grease a 10-inch oven-safe skillet with butter or spray with non-stick spray.
Make the Brownie Batter: Melt the butter, the 2 ounces of semisweet, and the 2 ounces of milk chocolate together in a large microwave-safe mixing bowl. Melt in 30 second intervals, stirring between each, until fully melted and smooth. Add the sugars, and whisk until just combined. Add the eggs and vanilla, and whisk until combined, then continue whisking for 2 minutes (this will create that crackly top on the brownies!). Add the flour, salt, cocoa, and espresso flour, and fold in with a spatula until the last streaks of flour just disappear into the batter. Add the chocolate chunks and gently mix in.
Bake: Spread the batter evenly in your prepared pan, and scatter any extra chocolate chunks over the top. Bake for 36 to 40 minutes. A toothpick inserted into the center should pull out moist crumbs, not wet batter, and should come out clean at the edges. (You can also use the tip of a knife to lift up a little piece from the center to see the doneness more clearly, if you have a hard time judging when brownies are done).
Serve + Store: Let the brownies rest about 10-20 minutes before digging in. This way, they set up just a smidge, but are still nice and gooey, while not being so hot they'll burn your mouth. Top with flaky sea salt and/or vanilla bean ice cream! Enjoy this straight from the pan while it's nice and warm. I think this is best while it's warm from the oven, while the chocolate is nice and melty, but leftovers can be stored in an airtight container at room temp for 3-4 days or in the fridge for 5-6.
Yield: The yield on this is totally up to you! Dig in with spoons for a date night treat. Or, spoon this onto plates to share with your family or friends. You'll likely end up with fewer servings this way. If you want to stretch the yield, slice it into 8 larger wedges, or into 12 to 16 smaller squares. This is rich so you can stretch the yield with smaller pieces if desired!