Step 1 of 9
Skillet Brownie
Preheat the oven to 350°F.
Step 2 of 9
Skillet Brownie
Spray an 8-inch cast-iron skillet with nonstick cooking spray.
Step 3 of 9
Skillet Brownie
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
Skillet Brownie
Add the granulated sugar, brown sugar, vanilla, and eggs to the chocolate mixture. Whisk vigorously until smooth and combined.
Step 5 of 9
Skillet Brownie
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
Skillet Brownie
Transfer the batter into the cast-iron skillet and smooth the top into an even layer.
Step 7 of 9
Skillet Brownie
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
Skillet Brownie
Check to see that brownie is done. Remove from oven or add time as needed.
Step 9 of 9
Skillet Brownie
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.