Preheat the oven to 350 degrees F.
Butter and flour three 8-inch round cake pans and line bottoms with parchment.
Cream together the butter and sugar. Add in the eggs, 1 at a time, mixing well after each.
Whisk together the cocoa powder, hot water and peppermint extract.
Whisk together the flour, baking soda, baking powder and salt.
Add 1/3 of the dry mixture into the creamed butter/sugar mixture. Mix. Add in 1/2 of the warm cocoa/water mixture. Mix again. Repeat till all the flour and cocoa are combined.
Divide batter evenly among the cake pans. Bake for 40 to 50 minutes till a toothpick inserted comes out clean.
For the buttercream: Cream together the butter and shortening. Gradually add in the powdered sugar. Add in the milk, vanilla, mint extract and desired amount of food coloring. Mix till combined.
To assemble: Roughly chop 1 box of Thin Mint cookies. Starting with the first layer of cake, spread about 1/4 cup of frosting across the top. Sprinkle 1/3 of the chopped cookies in the middle of the layer, leaving a 1/2 inch border around the edge. Top with the next layer of cake and repeat this process 1 more time then top with the last layer of cake. Cover the whole cake with the mint frosting. Using a medium star tip, pipe along the top edge of the cake with more mint frosting. Place whole Thin Mints along the edge on top of the piped frosting.