Preheat oven to 350°F Butter an 8-inch square baking pan. Line bottom with a strip of parchment paper, leaving an overhang on two sides; butter paper. Dust pan and sides of pan with cocoa, set aside. In a medium bowl, whisk together cocoa, flour, ginger, pumpkin-pie spice, and baking soda; set aside.
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. Transfer batter to prepared pan; smooth top.
Bake until a toothpick inserted in center comes out clean, 30-35 minutes. Transfer to wire rack; let cool completely. Using paper overhang, lift gingerbread from pan. Transfer to a cutting board and cut into 16 squares. Before serving, dust bars with confectioners' sugar, if desired.
To store, keep in an airtight container at room temperature, up to 3 days.