For the cake: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking pan with nonstick cooking spray and line the bottom with parchment.
In a large bowl, whisk together the flour, granulated sugar, baking soda, baking powder and salt. In a separate medium bowl, whisk together the buttermilk, speculoos, oil, vanilla, eggs and 3/4 cup water. Add the wet ingredients to the dry ingredients and stir to combine.
Pour the batter into the prepared pan and bake until a toothpick inserted into the center comes out clean. Begin checking for doneness at 28 minutes. Allow to cool completely.
For the frosting: In the bowl of a stand mixer fitted with a paddle attachment, mix together the butter and speculoos until creamy. Gradually add the powdered sugar and mix to combine. Mix in the vanilla and salt. Mix in the food coloring if desired until combined.
Use an offset spatula to frost the top of the cake in the pan. If desired, sprinkle the top of the cake with shredded coconut for snow, then decorate with rosemary sprigs for trees and mini candy canes.
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