Step 1: Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line the bottoms with parchment paper rounds and then grease and flour.
Step 2: In a large bowl, whisk together the sifted cake flour, granulated sugar, baking powder, and salt.
Step 3: In a separate medium bowl, whisk together the buttermilk, vanilla extract, and egg whites until just combined.
Step 4: Add the softened unsalted butter to the dry ingredients. Using a stand mixer with the paddle attachment (or a hand mixer), mix on low speed until the mixture resembles coarse sand, about 2-3 minutes.
Step 5: Gradually add the buttermilk mixture to the dry ingredients in three additions, mixing on medium-low speed for about 30 seconds after each addition, just until combined. Scrape down the sides of the bowl between additions. Do not overmix.
Step 6: Divide the batter evenly between the prepared cake pans. You can use a kitchen scale for precision if you want perfectly level layers.
Step 7: Bake for 30-35 minutes, or until a wooden skewer inserted into the center of the cakes comes out clean. The cakes should be lightly golden and spring back when gently pressed.
Step 8: Let the cakes cool in the pans on a wire rack for 10-15 minutes before inverting them onto the wire rack to cool completely. Cooling completely is crucial before frosting.
Step 9: FOR THE BUTTERCREAM: In a large bowl or stand mixer fitted with the paddle attachment, beat the softened unsalted butter on medium speed for 2-3 minutes, until light and fluffy.
Step 10: Gradually add the sifted powdered sugar, one cup at a time, mixing on low speed until incorporated. Increase speed to medium-high and beat for another 2-3 minutes until very light and fluffy. Scrape down the sides of the bowl as needed.
Step 11: Add the heavy cream and vanilla extract. Beat on medium-high speed for 1-2 minutes until the buttercream is smooth, creamy, and holds its shape. Adjust consistency with more cream or powdered sugar if needed.
Step 12: ASSEMBLE: Once both cake layers are completely cool, place one layer on your serving plate or cake stand. Spread about 1 cup of buttercream evenly over the top.
Step 13: Carefully place the second cake layer on top. Frost the top and sides of the entire cake with the remaining buttercream. Decorate as desired and serve!