To prepare a two tier celebration cake buttercream roses recipe, you will first need to prepare the cake. First, prepare your cake pan by lining the bottom of the pan with parchment paper. Then, preheat the oven to 350 degrees F (180 degrees C). Sift the flour, cocoa, baking powder and salt into the bowl of an electric mixer. Beat the mixture on medium speed until it's light and fluffy, about 2 minutes. Gradually add the sugar, beating well after each addition. Fold in the vanilla and the chocolate chips until the batter is completely smooth. Add the buttermilk and beat on low speed, scraping down the sides as needed, until all the ingredients are fully incorporated. In another bowl, beat the egg whites until they're stiff peaks, then gradually add to the dry ingredients, mixing just until combined. Scoop the frosting into piping bags and pipe the rose mixture onto the top of each cupcake. Pipe the icing onto each cake layer.