Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans. Line the bottoms with parchment paper rounds to ensure easy removal after baking.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside while you prepare the wet ingredients.

In the bowl of your stand mixer (or large bowl with hand mixer), beat the softened butter on medium-high speed until creamy (about 1 minute). Gradually add the granulated sugar and continue beating for 3-4 minutes until light, fluffy, and pale yellow. This extended creaming process incorporates air into the batter, helping create that perfect cake texture.

Add eggs one at a time, beating well after each addition until fully incorporated. Scrape down the sides of the bowl as needed to ensure even mixing. Beat in the vanilla extract until well combined.

With the mixer on low speed, add about one-third of the dry ingredients, followed by half of the buttermilk. Mix until just combined. Add another third of the dry ingredients, followed by the remaining buttermilk. Mix until just combined. Finally, add the remaining dry ingredients and mix only until no streaks of flour remain. Do NOT overmix as this will develop gluten and result in a tough cake.

Divide the batter evenly between the prepared pans. Smooth the tops with a spatula for even baking. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean and the cake begins to pull away from the sides of the pan.

Let cakes cool in the pans on a wire rack for 10-15 minutes before inverting them onto the rack to cool completely. Remove parchment paper carefully. Ensure cakes are completely cool before frosting (at least 1 hour, longer is better) to prevent the buttercream from melting.

In the clean bowl of your stand mixer (or large bowl with hand mixer), beat the softened butter on medium-high speed for 5-7 minutes until very pale, light, and fluffy. This step is crucial for achieving that smooth, creamy texture that makes the perfect birthday cake frosting.

With the mixer on low speed, gradually add the sifted powdered sugar, one cup at a time, until incorporated. This prevents a powdered sugar cloud and ensures smooth incorporation.

Add the vanilla extract and salt. Add the heavy cream, 1 tablespoon at a time, beating on medium-high speed until the frosting reaches your desired spreading consistency – light, fluffy, and smooth. Beat for another 2-3 minutes to incorporate air for that perfect fluffy texture.

If your cake layers domed, use a cake leveler or long serrated knife to carefully trim the tops for flat, even layers. This creates a more professional-looking finished cake.

Place one cake layer, top-side down (flat side up), onto your serving plate or cake stand. This gives you a flat surface to work with.

Spread about 1 cup of frosting evenly over the first layer using an offset spatula for a smooth finish.

Carefully place the second cake layer on top, top-side down to create a flat top surface for your cake.

Apply a thin, even layer of frosting over the entire cake (top and sides). This traps loose crumbs and creates a smooth base for your final coat. Chill the cake in the refrigerator for 15-20 minutes to set the crumb coat.

Apply the remaining frosting generously over the top and sides of the cake, using an offset spatula or bench scraper for a smooth or textured finish as desired. You can create decorative swirls or keep it elegantly smooth.

Add sprinkles, write "Happy Birthday" using a piping bag with a small round tip, pipe decorative borders, or decorate as desired. For a special touch, add edible flowers or chocolate decorations to make your cake truly stand out at the birthday party.