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Preheat oven to 350°F (177°C). Grease two 9-inch cake pans, line with parchment paper rounds (see #6 in Cake Baking Tips), then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. I recommend using nonstick spray for greasing.
Whisk the cake flour, baking powder, baking soda, and salt together in a large bowl. Set aside.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed for 3 full minutes. The creamed butter and sugar will be extra creamy. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Turn the mixer down to medium-high speed and beat in the 4 egg yolks one at a time, letting each egg yolk fully mix into the batter before adding the next. Beat in the vanilla extract until combined. Set aside.
Add about 1/3 of the dry ingredients and 1/2 of the buttermilk and beat on low speed until combined. Add 1/3 more of the dry ingredients and the rest of the buttermilk and beat on low speed until combined. Add the rest of the dry ingredients and beat on low speed until combined. The batter is very thick. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl.
Using a handheld or stand mixer fitted with a whisk attachment, beat the egg whites and cream of tartar together on high speed until fluffy peaks form, about 3 minutes. See photo and video above for a visual. Slowly and gently fold the egg whites into the thick cake batter. Avoid over-mixing as you don’t want to deflate the egg whites.
Pour/spoon batter evenly into cake pans.
Bake for around 27-31 minutes or until the cakes are baked through. After about 18 minutes, tent the cakes with aluminum foil to prevent the tops and sides from over-browning. To test the cakes for doneness, insert a toothpick into the center of the cake. If it comes out clean, it’s done. The cakes may look a little spongey on top as a result of the whipped egg whites. (That’s normal!)
Allow cakes to cool completely in the pans set on a wire rack. The cakes must be completely cool before frosting and assembling.
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cocoa powder, heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Do not over-whip. Add 1/4 cup more confectioners’ sugar or cocoa powder if frosting is too thin or another Tablespoon of cream/milk if frosting is too thick. Taste. Add more salt if needed. (I usually add another pinch.)
If cooled cakes are domed on top, use a large serrated knife to slice a thin layer off the tops to create a flat surface. This is called “leveling” the cakes. Discard or crumble over finished cake (or on ice cream!). Place 1 cake layer on your cake stand or serving plate. Evenly cover the top with frosting. Top with 2nd layer. (If desired, if the edges seem extra crumbly, apply a crumb coat which is a thin layer of frosting all over the cake. You can see I did this in the video tutorial above. Refrigerate the crumb coated cake for 15 minutes before adding the rest of the frosting.) Spread remaining frosting all over the top and sides. I always use an icing spatula and/or a bench scraper for the frosting. Garnish with sprinkles, if desired.
For extra neat slices, refrigerate cake for 30-60 minutes before slicing. This helps the cake hold its shape when cutting.
Cover leftover cake tightly and store at room temperature for up to 1 day or in the refrigerator for up to 5 days.