1. Preheat oven to 350 degrees. Brush a 9-by-13-inch baking pan with butter. Line bottom and two sides with parchment paper, leaving a 2-inch overhang on sides; butter paper, and set aside. In a medium bowl, whisk together flour, baking soda, cinnamon, and salt; set aside.

2. In a large bowl, whisk together sugar, butter, eggs, vanilla, and zest until smooth. Add flour mixture; mix just until moistened (do not overmix). Fold in pears, cranberries, and walnuts.

3. Transfer batter to prepared pan; smooth top. Bake until a toothpick inserted in center of cake comes out clean, 40 to 45 minutes. Cool completely in pan set on a rack. Using parchment overhang, lift cake from pan, and transfer to a cutting board. Cut into 18 squares; serve with whipped cream, if desired.