Preheat the oven to 350°F. Spray an 8x8-inch baking pan with flour-based baking spray.
In a large mixing bowl, use a hand mixer or stand mixer with a paddle attachment to beat the butter on medium speed for 30 seconds, until creamy.
Add the brown sugar and beat for an additional 30 seconds, until light and fluffy.
Mix in the molasses and sour cream for another 30 seconds, until incorporated.
In a separate bowl, sift together the flour, baking soda, ginger, cinnamon, nutmeg, cloves, allspice, and salt.
Working in three batches, gradually add the dry ingredients to the wet ingredients, mixing after each addition. The mixture will be thick and crumbly.
Add the hot water and mix for 1 minute, until the consistency is smooth.
Pour the cake batter into the prepared baking pan. Bake for 40 minutes, or until a toothpick inserted in the center comes out mostly clean; a few moist crumbs are fine.
Cool in the baking pan for 10 minutes then turn the warm gingerbread cake out onto a wire rack to cool completely.