How to Make It
Preheat oven to 350°F. Whisk together brown sugar, molasses, boiling water, and cubed butter in a medium bowl until butter melts. Whisk in baking soda and salt. Let stand until lukewarm, about 25 minutes. Whisk in egg.
Whisk together flour, ginger, cinnamon, nutmeg, allspice, cloves, and black pepper in a small bowl; add to brown sugar mixture, and whisk until smooth. Pour into a generously greased (with butter or cooking spray) and floured 9-inch square pan.
Bake in preheated oven until a toothpick inserted in center comes out clean, 20 to 25 minutes. Cool in pan 10 minutes. Transfer gingerbread to a wire rack, and cool completely, about 1 hour. (For the best texture, wrap tightly in plastic wrap or place in an airtight container, and let stand at room temperature overnight before serving.) Store in an airtight container up to 3 days.
Just before serving, sprinkle with powdered sugar, and cut into 9 squares. Serve with Brown Sugar-and-Ginger Whipped Cream.