Step 1

Arrange a rack in the center of the oven and preheat to 350 degrees F. Spray a 9-by-13-inch baking dish with nonstick spray.

Step 2

Melt the butter in a large saucepan over low heat. Add 1/2 cup of the chocolate chips and cook, stirring with a rubber spatula, until the chocolate is melted; remove from the heat. Stir in the sugar, cocoa powder and salt until combined. Stir in the eggs and then the flour until fully combined. Fold in the vanilla, zucchini, remaining 1/2 cup chocolate chips and the walnuts, if using.

Step 3

Scrape the batter into the prepared baking dish. Bake until the brownies are glossy and firm, 30 to 35 minutes. Let the brownies cool to room temperature or until slightly warm, at least 30 minutes (they will get fudgier the longer they cool). Cut into 24 pieces. Serve immediately or store in an airtight container for up to 1 week.