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1
Preheat the oven to 350u00b0F.
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2
Spray an 8-inch cast-iron skillet with nonstick cooking spray.
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3
Place butter and chopped chocolate into 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.
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4
Add the granulated sugar, brown sugar, vanilla, and eggs to the chocolate mixture. Whisk vigorously until smooth and combined.
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5
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.
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6
Transfer the batter into the cast-iron skillet and smooth the top into an even layer.
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7
Transfer the cast-iron skillet to the middle rack of the preheated oven.
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8
Bake the brownie for 28-32 minutes, until puffed at the edges and a toothpick inserted in the center comes out clean.
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9
Check to see that the cookie is done. Remove from oven or add time as needed.
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10
Allow the brownie to cool for 10 minutes before serving.
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11
Serve the brownie warm directly from the skillet, topped with vanilla ice cream. Alternatively, the brownie may be refrigerated in the skillet for 2 hours, then cut into slices.
