1. Preheat oven to 350 degrees. Butter an 8-inch square baking pan. Line bottom with a strip of parchment paper, leaving an overhang on two sides; butter paper. Dust paper and sides of pan with cocoa; set aside.
2. In a medium bowl, whisk together cocoa, flour, ginger, pumpkin-pie spice, and baking soda; set aside.
3. In a large bowl, whisk together butter, brown sugar, molasses, egg, and sour cream until smooth. Add flour mixture; stir just until moistened (do not overmix). Stir in chocolate chips.
4. Transfer batter to prepared pan; smooth top.
5. Bake until a toothpick inserted in center comes out clean, 30 to 35 minutes. Transfer to a wire rack; let cool completely. Using paper overhang, lift gingerbread from pan. Transfer to a cutting board, and cut into 16 squares.
6. Before serving, and once cool, dust bars with confectioners sugar, if desired. (To store, keep in an airtight container at room temperature, up to 3 days.)

Makes 16 bars.