Heat the oven to 350°F (175°C). Line a large baking sheet with parchment paper or a silicone baking mat. Alternatively, grease the baking sheet with butter and lightly coat it with flour to keep the dough from sticking.
In a medium mixing bowl, whisk together the all-purpose flour, baking soda, salt, ground cinnamon, ground ginger, allspice, cloves, and black pepper. Continue whisking until the spices and leavening are evenly distributed throughout the flour. Set the mixture aside.
Place the softened unsalted butter and packed brown sugar in a large mixing bowl. Using an electric mixer on medium-high speed, beat them together for approximately 2 minutes, or until thoroughly blended.
Add the eggs one at a time, beating after each addition until completely incorporated. Mix in the vanilla extract and unsulfured molasses, continuing until the mixture is uniform.
Gradually add the prepared dry mixture to the wet ingredients. Stir just until everything is combined and a firm, stiff dough develops. Ensure no dry flour remains, but avoid excessive mixing.
Divide the dough into two equal portions. With well-floured hands, form each portion directly on the prepared baking sheet into a slightly flattened log measuring approximately 12 inches long and 2 inches wide. Flour your hands and the working surface generously as needed to prevent the sticky dough from adhering.
Sprinkle the tops of both shaped logs with 1–2 tablespoons of powdered sugar.
Place the baking sheet in the preheated oven and bake the logs for about 30 minutes, or until they feel slightly firm when gently touched.
Remove the baking sheet from the oven and allow the logs to rest for 5–10 minutes before slicing.
Carefully transfer the partially cooled logs to a cutting board. Using a sharp serrated knife, cut each log crosswise into slices approximately 3/4 inch thick.
Arrange the sliced biscotti on the baking sheet with their cut surfaces facing downward. Return them to the oven and bake for approximately 10 minutes, or until they become crisp.
Transfer the finished biscotti to a wire rack and allow them to cool completely before serving or storing.