Preheat oven to 350°.
Spray a quarter sheet pan (or 9 x 13 inch baking dish) with baking spray, line the pan with parchment paper and then spray the parchment paper. This will ensure the cake will be easily removed from the pan without sticking.
Pour the buttermilk into a measuring cup, add baking soda, vanilla, and egg and whisk together. If you don’t have buttermilk you can use regular milk. Just remove 1 teaspoon of the milk and add 1 teaspoon of vinegar. Let that set for a few minutes before adding the rest of the ingredients.
In large bowl combine flour, sugar, and salt and whisk together.
Heat the butter in a small pan over medium low heat. When the butter is melted add the cocoa powder and whisk to combine. Add the water and bring to a soft boil, reduce the heat and simmer for one minute. Add to the flour mixture and stir together. Let cool for a few minutes.
Add the milk and egg mixture and stir until smooth.
Pour batter into the prepared pan and bake for 15-20 minutes, or until the cake bounces back when lightly poked with your finger. Cool completely on the counter, then refrigerate for at least one hour. The shapes are even easier to cut out if you let the cake chill in the fridge over night.
Remove the cake from the pan and peel away the parchment paper. Trim the edges of the cake so you have a nice flat surface to deal with. Get out your cookie cutters and start cutting out shapes.
After cutting out all of the shapes, crumble whatever scraps you have left into a bowl. Add a small amount of icing or frosting and begin to stir until a thick cakey mixture forms…adding more frosting as needed. You want to add enough so that you can scoop the mixture out and roll in into a ball.
Start decorating! Get out whatever decorating items that you have…sprinkles, powdered sugar, colored sanding sugar, etc. Frost the cakes or dust with powdered sugar…or both, and top with your decorations. Get creative and have fun with it!!