Step 1

Preheat oven to 350 degrees F. Line an 8-inch-square baking pan with foil, letting it overhang on the sides; coat with cooking spray.

Step 2

Melt butter in a large saucepan over medium-low heat. Remove from heat, add unsweetened chocolate and stir until melted. Add sugar and salt; stir until well combined. Stir in zucchini and vanilla. Add eggs, one at a time, vigorously mixing after each addition. Fold in the flour until almost incorporated. Fold in chopped chocolate (or chips) and 1/2 cup pecans (if using) until just combined. Transfer the batter to the prepared pan, spreading evenly. Sprinkle with the remaining pecans (if using).

Step 3

Bake until the sides look dry and the middle is firm to the touch, about 30 minutes. Let cool completely, then lift out of the pan using the foil. Cut into 16 squares.