Prepare the Chocolate Batter: Beat the 30 g butter and 50 g sugar together until light and fluffy. Add a dash of vanilla, a pinch of salt, the egg yolk, and baking soda, then continue beating for a few minutes. Sift in the cake flour and Dutch cocoa powder, add the buttermilk or kefir, and stir just until the mixture is combined. Transfer Chocolate Batter: Set the chocolate mixture aside until you are ready to assemble the cakes. Transfer it to a small piping bag so it will be ready for piping between the layers of white cake batter.
Prepare the Cake Pans: Preheat the oven to 325°F. Butter and flour two 8-inch round cake pans.
Cream the Main Ingredients: Beat the 226 g softened butter, 400 g sugar, and 75 g coconut oil together until the mixture is light and fluffy, about 5 minutes.
Add the Egg Whites: Add the 4 egg whites one at a time, beating for about 2 minutes after each addition. Scrape down the sides and bottom of the bowl as needed.
Add the Flavorings: Add the sea salt, vanilla extract, sakura extract, and baking powder. Beat well, then scrape down the bowl again to ensure the ingredients are evenly incorporated.
Add the Flour: Sift in the cake flour. Fold it in slightly with a spatula first to prevent flour from scattering when the mixer is turned on.
Add the Buttermilk: With the mixer running on low, slowly pour in the 240 g buttermilk or kefir. Mix only until the batter is just combined.
Fold in the Sprinkles: Add the pink sprinkles and fold them through the batter. Scrape the bottom of the bowl to make sure the sprinkles are evenly distributed and the batter is homogeneous.
Create the First Cake Layer: Divide enough white cake batter between the prepared pans to create a layer about ½ inch thick in the bottom of each pan. Smooth the batter gently.
Pipe the Chocolate Branches: Snip a small opening at the tip of the piping bag containing the chocolate batter. Pipe loose, overlapping squiggles across the white batter, gently shaking the bag as you move. Avoid straight lines and create intertwined, branch-like patterns. Do not add excessive chocolate batter, as too much can weigh down the center of the cake. Build the Remaining Layers: Add more white cake batter over the chocolate squiggles and smooth the surface very gently so the pattern is not disturbed. Pipe another layer of chocolate squiggles over the batter.
Distribute the Batter: Continue alternating white cake batter and chocolate squiggles until all the batter is divided evenly between the two prepared pans.
Bake the Cakes: Bake the cakes at 325°F for approximately 40–50 minutes, until baked through.
Cool the Cakes: Allow the cakes to cool in their pans for 5 minutes. Then invert them onto a cooling rack.
Beat the Buttercream Base: Beat the room-temperature butter until it becomes very light in color, about 7 minutes.
Flavor the Buttercream: Add the vanilla extract, sakura essence, and sea salt. Beat until the ingredients are fully combined.
Add the Powdered Sugar: Sift in the powdered sugar and beat until the buttercream is combined and fluffy.
Finish the Buttercream: Add the heavy whipping cream or milk and beat again until fully incorporated.
