This recipe comes from our book "Clean Slate: A Cookbook and Guide."
Preheat oven to 350 degrees F. Brush an 8-inch square baking pan with oil. Line with parchment, leaving overhang. Brush parchment with oil. Melt half of chocolate in a heatproof bowl; whisk in oil, sugar, and syrup. Remove from heat. Whisk in eggs, one at a time, then cocoa, baking soda, and salt. Fold in flour and remaining chocolate. Pour batter into prepared pan; sprinkle with walnuts. Bake until a toothpick comes out clean, about 30 minutes. Let cool completely on a wire rack. To serve, cut into 2-inch squares.