Preheat your oven to 350°F (175°C) and prepare your cake pans by greasing them and lining with wax paper.

In a large mixing bowl, sift together flour, cocoa, sugar, baking soda, cream of tartar, and salt.

Add buttermilk and butter to the dry mixture; mix on medium speed for about 2 minutes. Incorporate eggs and vanilla, mixing until smooth.

Divide the batter evenly across the prepared pans and bake for 30-45 minutes, until a toothpick comes out clean.

Let the cakes cool in the pans for 10-15 minutes before transferring them to a wire rack to cool completely.

Beat together softened butter and shortening until creamy. Gradually add sifted confectioners' sugar and milk until desired consistency.

Once cakes are cool, cut mini layers with a cookie cutter and stack them with frosting in between.

Frost the outsides of each stacked cake and decorate with pastel colors or toppings like mini chocolate eggs or jelly beans.