Preheat oven to 350 degrees. Spray 2 8-inch round cake pans with cooking spray and line the bottoms with parchment paper. Spray the paper with cooking spray and lightly dust with flour. Set pans aside.

Melt the butter in a medium saucepan set over medium heat. Add the water and bring mixture to a boil, stirring occasionally. Remove from the heat and use a rubber spatula to stir in the chopped chocolate; continue stirring until the chocolate is fully melted.

Pour the mixture into a medium mixing bowl and allow to cool for 10 minutes, then stir in the buttermilk, eggs, and vanilla. Set aside.

Combine the flour, sugar, baking soda, and salt in a large mixing bowl and whisk to combine. Gradually stir in the chocolate mixture; stir until well combined, taking care to scrape up from the bottom of the bowl. Divide batter evenly between the prepared pans.

Bake 26 to 32 minutes, until a cake tester inserted into the center of the cakes comes out clean. Cool cake layers in pans on wire racks for 15 minutes, then carefully invert the onto the racks to cool completely before frosting.

Place the chocolate in a microwave-safe bowl and heat at 50% power in 15-second intervals, stirring after each interval, until chocolate is melted and smooth. Set aside to cool.

Place the butter in a large mixing bowl and beat with an electric mixer until light and creamy, about 1 minute. With mixer on low, beat in 3 1/2 cups of the powdered sugar, 2 tablespoons of the milk, and the vanilla and salt until well combined.

Add the cooled white chocolate and mix on low until incorporated. Turn mixer to medium and beat until frosting is very creamy, scraping down the sides and bottom of the bowl as necessary. Add additional powdered sugar to thicken frosting or additional milk to thin it out to a spreadable consistency.

Set one cooled cake layer on a cake stand or cardboard cake circle and spread a layer of frosting over the top. Top with the second cake layer and frost the cake with the remaining frosting. Decorate as desired and serve.