Preheat the oven to 350°F (177°C). Grease two 9-inch round cake pans, line with parchment paper rounds, then grease the parchment paper. Parchment paper helps the cakes seamlessly release from the pans. (If it’s helpful, see this parchment paper rounds for cakes video & post.) I recommend using nonstick spray for greasing.

In a medium bowl, whisk the cake flour, baking powder, baking soda, and salt together. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on medium-high speed until light and creamy, about 3 minutes. Scrape down the sides of the bowl with a silicone spatula as needed. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the oil and beat for 1 minute until combined. Beat in the egg yolks until incorporated, then beat in the vanilla extract.

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 will be very thick. You may need to whisk it all by hand to make sure there are no lumps at the bottom of the bowl. Set aside.

In another large 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, medium-stiff peaks form, about 3 minutes.

Fold the whipped egg whites into the cake batter. Work slowly and gently, as you don’t want to deflate the egg whites. Pour/spoon the batter evenly into the prepared pans.

Bake for 27–31 minutes or until a toothpick inserted in the center of the cake comes out clean. Cool the cakes in the pans set on a cooling rack for 1 hour. Run a knife around the edges to loosen the sides, remove the cakes from the pans, peel off the parchment, and place on the rack to finish cooling. The edges of the cakes can be crumbly; this is normal. The cakes must be completely cool before frosting and assembling.

In a large bowl 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, 4 Tablespoons of 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 (30g) more confectioners’ sugar if frosting is too thin or 1–2 more Tablespoons of cream if frosting is too thick. Taste. Add another pinch of salt if desired.

(For additional help with this step, see this how to assemble a layer cake video & post.) First, using a large serrated knife, slice a thin layer off the tops of the cakes to create a flat surface. Discard (or crumble over ice cream!). Place one cake layer on your cake stand, cake turntable, or serving plate. Spread about 3/4–1 cup (180–240g) of frosting in an even layer on top. Top with the second cake layer and spread about 1 cup (240g) of buttercream in a very thin layer over the top and sides to make the crumb coat. If desired, run a bench scraper around the cake to help smooth out the frosting on the sides. Refrigerate the cake until the crumb coat has set, about 20 minutes. Finish by spreading the top and sides with the remaining buttercream. Decorate the cake with sprinkles, if desired. For extra neat slices, refrigerate the 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. If the cake has been stored in the refrigerator, let the cake sit out at room temperature for at least 30 minutes to soften up again before serving.