Step 1 of 9
Preheat the oven to 350°F.
Step 2 of 9
Spray an 8-inch cast-iron skillet with nonstick cooking spray.
Step 3 of 9
Place butter and chopped chocolate in a large microwave-safe mixing bowl. Microwave on high in 30 second increments, stirring between each, until the chocolate is fully melted. Allow the mixture to cool for 5 minutes.
Step 4 of 9
Add the granulated sugar, brown sugar, vanilla, and eggs to the chocolate mixture. Whisk vigorously until smooth and combined.
Step 5 of 9
Add the flour, cocoa, baking soda, salt, and espresso powder, if using, to the chocolate mixture. Stir vigorously with a rubber spatula until a thick batter forms and no lumps remain. Fold in the chocolate chips.
Step 6 of 9
Transfer the batter into the cast-iron skillet and smooth the top into an even layer.
Step 7 of 9
Bake the brownie on middle rack of oven until puffed at the edges and a toothpick inserted in the center comes out clean, 28-32 minutes.
Step 8 of 9
Check to see that brownie is done. Remove from oven or add time as needed.
Step 9 of 9
Allow the brownie to cool for 10 minutes before serving. Serve the brownie warm directly from the skillet, topped with vanilla ice cream. Alternately, the brownie may be refrigerated in the skillet for 2 hours, then cut into slices.