Make the gingerbread sheet cake first, so it has time to cool. Preheat the oven to 350°F/175°C. Line a 9x13-inch (quarter sheet) pan with parchment paper and spray with nonstick spray.
Place two flower nails or heating cores in the center of the pan (if you have them on hand), with the flat side resting against the cake pan. Make sure they are spaced a few inches apart and spray them thoroughly with non-stick spray.
In a large bowl, whisk 3 1/2 cups all-purpose flour, 2 Tbsp ginger, 1 Tbsp cinnamon, 1 Tbsp baking powder, 1 1/2 tsp baking soda, 3/4 tsp cloves, and 1 tsp salt together. Set aside.
In a separate, large bowl, vigorously whisk together 2 3/4 cup dark brown sugar and 4 large eggs for a minute, until smooth. I usually make this cake recipe by hand, but you can also use a hand mixer or stand mixer. Beat on a medium-high speed for a minute with an electric mixer. The mixture should become lighter in color as air is incorporated.
Then mix 1 cup warm water, 1 cup buttermilk, 3/4 cup vegetable oil, 1/2 cup molasses, and 1 tsp vanilla into the sugar /egg mixture. Whisk together until combined.
Add the dry ingredients into the wet ingredients and whisk together until just combined.
Pour the batter into the prepared sheet pan. Reposition the flower nails if they've shifted so that they’re evenly spaced across the pan.
Bake for 50-55 minutes, or until a toothpick inserted into the middle of the cake comes out with a few moist crumbs. If you don't use heating cores, the cake will take 5-10 additional minutes to bake. Rotate the pan halfway through to help the cake bake evenly.
Let the cake cool in the pan for about 30 minutes, then carefully remove it from the pan to a wire rack to finish cooling.