1. Preheat oven to 350°F (176°C). Grease the sides of a 9×13 inch cake pan and place parchment paper in the bottom.
2. In a medium sized bowl, combine the flour, baking soda, baking powder and salt.
3. In a large mixer bowl, cream the butter and sugars together until light and fluffy, about 3-4 minutes.
4. Add the eggs one at a time, mixing just until combined before adding the next.
5. Add the vanilla extract and mix until well combined.
6. Add the dry ingredients in two batches, mixing until combined. Dough will be thick and sticky.
7. Stir in the chocolate chips.
8. Press the dough evenly into the cake pan. I use my offset spatula to spread it around in the pan, since it’s sticky.
9. Bake for 15-20 minutes or until the top of the cookie is lightly golden.
10. Remove from oven and allow to cool completely in the cake pan, then transfer to serving plate. I find it helps to refrigerate the cake only after it has cooled to firm it up before removing from the pan. It helps it stay together while you remove it from the pan. To remove this from such a large pan, I use a cooling rack and a cardboard cake rectangle (two cooling racks would also work). I place the cooling rack against the top of the cake pan, then flip it over. Immediately and quickly place the cardboard cake rectangle on top of the cookie cake (which is the bottom) and flip the whole thing over using the cooling rack. Remove the cooling rack from the top of the cake.
9. To make the frosting, beat the shortening and butter until smooth.
10. Slowly add half of the powdered sugar. Mix until combined and smooth.
11. Add the vanilla extract and 3 tablespoons of water or milk and mix until smooth.
12. Add the rest of the powdered sugar and mix until smooth. Add more water to get the right frosting consistency.
13. Divide the frosting into 4 bowls – a little less than half in one bowl for green, about 1/4 cup in another bowl for brown and a little less than 1/4 cup in another bowl for yellow. The remaining frosting will all be blue.
14. After you’ve colored your frostings, spread a layer of blue frosting over the top of the cake and smooth it out as nice as you can.
15. To make the trunk of the tree, pipe 5 small lines at the “base” of what will be the tree. I used piping tip Wilton 32.
16. Pipe the green frosting into the shape of a Christmas tree. I used Ateco tip 844. Think of piping a shell border, but with long tails and make the shells side by side. Start with one row of 9 across, then 8, then 7 and so on until the top row is 4 shells across. If you’ve like to practice before adding this to the top of the cake, you can pipe it onto parchment paper and then reuse the frosting to pipe it onto the cake. You could also pipe the tree onto the parchment paper, freeze it and then peel off the parchment paper and transfer the buttercream tree onto the cookie.
17. Top the tree with a yellow star. I used Wilton tip 21. Again, you could pipe the star right onto the cookie, or onto parchment paper and freeze it.
18. Decorate the tree with the sprinkles or candies of your choice. I used sprinkles, but Christmas colored M&Ms would also make a nice option. You can also add sprinkles to the blue frosting to make “snow”, if you’d like.
19. Store in an airtight container until ready to serve.