Prepare Pan and Dry Ingredients: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line with parchment paper leaving an overhang on the sides for easy lifting. In a medium bowl, whisk together the all-purpose flour, baking soda, salt, ground ginger, cinnamon, cloves, nutmeg, and allspice. Set aside.
Cream Butter and Sugar: In a large bowl, using an electric mixer, cream together the softened unsalted butter and packed light brown sugar until light and fluffy, about 2-3 minutes. Scrape down the sides of the bowl as needed.
Add Eggs and Molasses: Beat in the large eggs one at a time, mixing well after each addition. Stir in the dark molasses until fully combined. The mixture may look slightly curdled at this point, which is normal.
Combine Wet and Dry Ingredients: Gradually add the dry ingredient mixture to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix on low speed until just combined, being careful not to overmix. Overmixing can lead to dense Gingerbread Cake Bars.
Bake the Cake Bars: Pour the batter into the prepared 9x13 inch baking pan and spread evenly. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, not wet batter. Avoid overbaking to prevent dry bars. Let the Gingerbread Cake Bars cool completely in the pan on a wire rack before frosting.
Prepare the Cream Cheese Frosting: While the bars cool, prepare the frosting. In a medium bowl, beat together the softened cream cheese and softened unsalted butter until smooth and creamy. Gradually add the powdered sugar, mixing until well combined. Stir in the vanilla extract. If the frosting is too thick, add milk or cream one tablespoon at a time until it reaches a spreadable consistency.
Frost and Serve: Once the Gingerbread Cake Bars are completely cooled, spread the cream cheese frosting evenly over the top. You can sprinkle with a pinch of cinnamon or chopped candied ginger for garnish. Cut into squares or rectangles and serve immediately, or store as directed.