Preheat your oven to 350°F (175°C) for even baking.
In your skillet, melt the chocolate chips and butter over low heat, stirring until smooth.
Remove from heat and stir in the sugar and vanilla until well combined.
Add the eggs one at a time, mixing well after each addition.
In a separate bowl, whisk together cocoa powder, flour, and salt, then gradually add to the wet ingredients.
Fold in walnuts if using, then spread the batter evenly in the skillet.
Bake for 20-25 minutes, until a toothpick inserted comes out with a few moist crumbs.
Let cool for a few minutes before slicing. Serve warm with vanilla ice cream.
