Preparation

• Preheat your oven to 350°F (175°C).
• In a microwave-safe bowl, microwave 1 ½ cups of the chocolate chips in 60-second intervals, stirring between each, until fully melted.
• Whisk the melted chocolate with the melted butter and granulated sugar until smooth.
• Add the vanilla extract and eggs, and whisk until incorporated; the mixture may tighten or seize slightly—that’s normal.
• Sprinkle the unsweetened cocoa powder, flour, and salt over the wet mixture and gently stir until just combined.
• Fold in the optional chopped walnuts with a spatula.
• Spray an 8-inch cast-iron skillet with nonstick spray and pour in the batter, spreading it evenly.
• Sprinkle the extra ¼ cup of chocolate chips atop the batter.
• Bake for about 30 minutes, until the edges are set and the center remains fudgy.
• Let cool in the skillet for 5–6 minutes, then serve warm with vanilla ice cream or whipped cream if desired.