Preheat your oven to 325 degrees. Spray a 9×13 metal pan on the bottom, line with parchment paper, and spray again this time making sure to get the sides. If you need to use metal binder clips to hold down the sides, remember to use metal, not plastic. Set aside.
In a medium size bowl, whisk together the baking powder, ground ginger, nutmeg, ground cinnamon, cloves, and all-purpose flour. Set aside.
In a stand sized mixer fitted with the paddle attachment, cream the room temperature butter, dark brown sugar, and granulated sugar on medium speed until light and fluffy. About 5 minutes.
With the mixer on low speed, add the molasses and vegetable oil. Mix just until combined. With a rubber spatula, scrape down the sides and bottom of the bowl.
Add the eggs, one at a time, and mix on medium speed for 20-30 seconds after each addition. Scrape with a rubber spatula after each addition.
Add the vanilla extract to the creamed mixture. Mix on low until smooth.
Scrape the bottom of the bowl and gradually add a third the dry ingredients followed by half of the buttermilk. Mix on low just until combined. Repeat this step until you end on the flour mixture.
Pour the batter into the prepared sheet cake pan and smooth the top with a spatula.
Bake in the preheated oven for about 45-50 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Mine took the full 50 minutes.
Let it completely cool in the cake pan. While the cake cools, make the frosting
For the buttercream
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium-high until creamy, about 3 minutes. With a rubber spatula, scrape down the sides of the bowl.
Once the butter is creamy, add the brown sugar and beat on medium-high until combined, about 1 minute.
Add the powdered sugar, heavy cream, GINGERBREAD FLAVORING, and salt. Turn the mixer to low and mix just until combined, about 30 seconds. Turn the mixer up to medium-high and beat until completely combined and creamy.
Assembly
Once the cake has cooled, keep the cake in the pan, and with a piping bag and Wiltons piping top 1M, pipe back and fourth starting from top and moving down the cake. Sprinkle with gingerbread people (I got mine from trader joes) crumbs and garnish with extra!