Place the white chocolate (broken into pieces) and hot water in a microwave-safe bowl.
Melt the white chocolate in 20 second intervals, stirring between each time, until the chocolate is melted. Set aside.
Separate the eggs, placing the egg yolks in a small bowl to set aside for later and place the egg whites in a mixing bowl. Beat the egg whites until they are frothy and hold stiff peaks. Set the frothed egg whites in the refrigerator for later.
Preheat the oven to 325 degrees Fahrenheit.
In the bowl of a stand mixer, cream together the butter and sugar until light and fluffy.
Add in the egg yolks and vanilla. Mix well.
Add in the melted white chocolate and mix well.
In a small bowl, combine the flour, salt and baking soda.
Add the flour mixture to the butter/sugar alternately with the buttermilk, beating and scraping the sides of the bowl as needed between each addition.
Carefully fold in the egg whites and mix just until the egg whites are fully mixed in.
Grease with shortening and flour two 9" cake pans.
Divide the cake batter evenly between the two prepared pans.
Bake at 325 degrees Fahrenheit for about 25-28 minutes or until the center of the cake bounces back when you touch it.
Allow the cakes to cool for 10 minutes, then turn them out onto a wire rack to cool completely.
Prepare the Frosting:
**Melt the white chocolate in advance and allow it to come to room temperature. If it is hot when you add it to the whipped butter, it will melt the butter and the texture of the frosting will be off.
Using a stand mixer, beat the butter for 2 minutes until it is almost white in appearance. Drizzle in the white chocolate, mixing and scraping the bowl as needed as you pour it in.
Add the powdered sugar and vanilla.
Beat the frosting for 2-3 minutes until the frosting is smooth and creamy.
Assemble the Cake:
Place one cooled cake layer on a cake plate.
Frost the top of the cake layer with frosting.
Place the second cake on top.
Frost the outside of the cake, both the top and the sides, with the rest of the frosting, smoothing it was you go.
Decorate the top with additional shaved white chocolate or fresh fruit if desired.
This cake can be stored at room temperature for 24 hours, then refrigerate any leftovers.
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Notes
The calories shown are based on using all of the frosting and the cake being cut into 15 pieces, with 1 serving being 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.