01 - Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a medium bowl, combine flour, brown sugar, ginger, cinnamon, cloves, nutmeg, salt, and baking soda. Cut in cold butter using a pastry blender or fingertips until coarse crumbs form. Stir in molasses until mixture clumps. Set aside.
03 - In a large bowl, whisk together flour, granulated sugar, baking powder, baking soda, and salt. In a separate bowl, whisk together buttermilk, vegetable oil, egg, and vanilla extract until smooth. Add the wet ingredients to the dry ingredients and mix gently until just combined; avoid overmixing.
04 - Spread the cake batter evenly into the prepared baking pan. Evenly sprinkle the gingerbread cookie crumble over the cake batter.
05 - Bake for 32–36 minutes, or until a toothpick inserted into the center comes out clean and the crumble topping is golden brown.
06 - Allow the bars to cool completely in the pan before lifting out and cutting into 12 individual portions.