Preheat oven to 350°F. Grease and flour a 9x13 inch baking pan.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and brown sugar on medium high speed until light and fluffy. Add the molasses and mix until incorporated. Add the eggs one at a time, mixing after each.
In a separate bowl, combine the flour, ginger, cinnamon, nutmeg, baking soda, and orange zest. In another small bowl, whisk together the orange juice and the buttermilk. Add the dry ingredients to the mixer in 3 additions, alternating with the buttermilk mixture, and ending with the dry ingredients. Mix until just combined.
Pour batter into the prepared baking pan and bake until a toothpick inserted into the center comes out clean, 40-45 minutes. Let cool on a wire rack. Serve with whipped cream, if desired.