Preheat the oven to 300 degrees and grease two 9-inch cake pans well. In a large bowl, combine dry ingredients (flour through salt). Make a well in the center of the dry mixture and add egg, egg white, milk, and vegetable oil. Mix to combine. Add vanilla and boiling water and mix until well incorporated.
Divide batter evenly between the two pans and bake 30-35 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.
Make the mint frosting:
While the cakes cool, mix softened butter, shortening, and powdered sugar until smooth. Add water or milk, vanilla, and peppermint extract and mix again. Slowly add food coloring to your liking (I’d say go with about 2/3 green and 1/3 yellow) and mix until color is even.
Place one layer of the cake on a platter and frost the top. Top with second layer of cake and frost the entire cake, reserving about 1 1/2 c. for extra frosting garnishes.
To get the look pictured, use a cake decorating tip like Wilton 1M to pipe swirls around the outer edge of the cake’s surface. Insert whole Andes mints into the swirls, press them into the base of the cake, or garnish however you like!