• Finely crush the walnuts using a Magic Bullet or food processor. Remove any large pieces. Measure out 1 Tbsp plus 1 tsp into a small bowl. Set aside the remaining crushed walnut pieces.

For the Cake

• Heat oven to 350℉
•  Line the bottom of the 4-inch cake pan with parchment. Brush or spray with non-stick spray and flour the pan.
• Place the egg white in a 1-cup liquid measuring cup. (See Note) Beat until it forms stiff peaks, about 1 minute. Set aside.
• In a 2-cup glass measuring cup, add the egg yolk, sugar, and vanilla. Beat until light lemon-colored, about 20 seconds.
• Add the baking powder and cake flour, alternating with the water. Add the 2 Tbsp + 1 tsp of crushed walnuts. Mix. Fold in 1/4 of the whipped egg whites lightly so you don’t deflate them. Add remaining egg whites and fold until combined. Don’t overmix.
• Place all the batter in the prepared cake pan. Put in the oven and bake for 20-25 minutes. When done, the top will be very light brown. A toothpick will come out clean.
• Remove to a cooling rack. Let cool in the pan for 10 minutes. Remove gently and let it cool completely. Split the cake in half so you have 2 even layers. Set aside.

For the crème frosting and filling

•  Mix all the crème filling ingredients in a 1-cup liquid measuring cup. Whisk until combined. Refrigerate for 5 minutes.

Assembly

• Spread 1/2 of the filling between cakes. Place the remaining crème filling on top of the cake. Combine the grated chocolate and remaining crushed walnuts. Use the mixture to top the cake.
•  Refrigerate the cake until ready to serve and any leftovers.