Preheat the oven: Set your oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper to prevent sticking and ensure easy removal. Consider using non-stick baking spray for an extra layer of protection.
Mix dry ingredients: In a large bowl, whisk together flour, baking powder, baking soda, and salt. Set aside. This ensures even distribution of the leavening agents, which is crucial for the cake to rise properly. A sifter can be used to aerate the flour, providing a lighter texture.
Cream butter and sugar: In a separate bowl, beat the softened butter and sugar together until light and fluffy, about 3-5 minutes. This step incorporates air, ensuring the cake is light and tender. Use a stand mixer for efficiency, especially for large batches.
Add eggs and vanilla: Beat in the eggs one at a time, followed by the vanilla extract, mixing until well combined. Ensure each egg is fully incorporated before adding the next to maintain a smooth batter. This technique prevents curdling and ensures a silky batter.
Combine wet and dry ingredients: Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Start and end with the dry ingredients, mixing until just combined to avoid over-mixing, which can make the cake dense. A folding technique can assist in gently incorporating the ingredients.
Bake the cakes: Divide the batter evenly between the prepared pans. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean, indicating the cake is fully baked. Rotate the pans halfway through to promote even baking.
Prepare buttercream frosting: Beat the butter until creamy. Gradually add powdered sugar, heavy cream, and vanilla, beating until fluffy. Divide frosting into bowls and tint with pastel food coloring to create a vibrant palette. Adjust consistency with extra cream for smoother spreading.
Cool and frost: Allow cakes to cool in pans for 10 minutes, then transfer to a wire rack to cool completely. Frost with pastel buttercream, creating an ombre effect if desired for an elegant finish. Use an offset spatula for even layers of frosting.
Decorate: Top the cake with edible flowers and gold leaf for an elegant touch. Place the 21 cake topper as the finishing touch, making it a centerpiece worthy of a 21st birthday celebration. For added visual interest, consider arranging the gold leaf in a pattern.
