Mint Oreo Cake

Ingredients :
Devils Food Cake:
3/4 cup + 2 tbsp all-purpose flour
3/4 cup + 2 tbsp granulated sugar
1/3 cup + 1 tbsp unsweetened cocoa powder 
1/2 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
1/2 cup buttermilk
1/4 cup vegetable oil 
1 large egg, at room temperature
1/2 teaspoon pure vanilla extract
1/2 cup boiling water (or coffee)

Mint Cookie & Cream Buttercream:
3/4 cup (1.5 sticks) unsalted butter 
1/2 cup shortening 
4 1/2 – 5 cups confectioners sugar
1/4-1/3 cup half & half or heavy cream
1/8 tsp salt (or to taste)
1 tsp peppermint extract, or to taste 
1/2 cup crushed Oreos or other chocolate sandwich cookies  

Chocolate Ganache:
1/2 cup semisweet chocolate chips 
1/3 cup heavy cream 

Garnish:
1/2 cup reserved mint buttercream 

Crushed Oreos 
4-5 Oreos cut in half 

Instructions :
To make the devil cake:
1. Preheat oven to 350°F. Grease and flour two 6×2-inch cake pans, and line the bottom of each with a round of parchment paper (if desired).
2. Sift the flour, sugar, cocoa powder, baking powder, baking soda and salt into a medium bowl. Set aside.
3. In a large bowl, beat buttermilk, oil, room temperature eggs, and vanilla on high speed using a handheld mixer or stand mixer until combined. Slowly add the dry ingredients to the wet ingredients while turning the mixer to low. Add boiling water (or coffee). The batter will be thin. It’s ok.
4. Pour the batter evenly into the prepared pan and bake for about 25 minutes, or until a toothpick inserted in the center comes out clean and the cake springs back when touched. Cool in pan for 10-15 minutes, then transfer to a wire rack. Let cool completely before freezing.
Make Peppermint Cookies and Buttercream:
5. In a large bowl, using a hand mixer or stand mixer fitted with the paddle attachment, beat the butter and fat on medium speed until creamy – about 2 minutes. Add 4 1/2 cups powdered sugar, 1/4 cup heavy cream, peppermint extract and salt with mixer on low. Increase to high speed and beat for a full 3 minutes. If the frosting is too thin, add more powdered sugar, and if the frosting is too thick, add a little cream (usually up to the recommended 1/3 cup). Keep in mind that adding crushed Oreos will thicken the icing consistency.
6. Take about 1/2 cup of frosting and place in a piping bag fitted with a Wilton 1M or 2D star tip. Set aside.
7. Add 1/2 cup crushed Oreos to remaining cream and beat until well combined.