Let’s embark on creating this elegant White Chocolate Cake. First, prepare your pans and the white chocolate for the cake. Preheat your oven to 350°F (175°C). Thoroughly grease and flour either two or three 9-inch round cake pans. Line the bottoms with parchment paper circles and set them aside.

Next, carefully melt the white chocolate for the cake batter. Break or chop the 8 oz. of white chocolate baking bars into small, uniform pieces and place them in a microwave-safe bowl. Heat in the microwave at 50% power in 30-second intervals, stirring well after each interval. Continue until the chocolate is melted and smooth. This usually takes 1 to 1 1/2 minutes total. Crucially, do not overheat white chocolate, as it can seize (become thick and grainy) very easily. Once smooth, set it aside to cool for about 10 minutes while you prepare the rest of the batter. It should still be liquid but not hot.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. This distributes the leavening and salt evenly. Set this dry mixture aside.

In a large bowl (or the bowl of your stand mixer with the paddle attachment), beat the softened unsalted butter on medium speed for about 30 seconds until smooth. Gradually add the granulated sugar and continue beating on medium speed until the mixture is well combined and slightly fluffy (about 2-3 minutes). Reduce the mixer speed to low. Add the room temperature eggs one at a time, beating well after each addition until fully incorporated. Mix in the vanilla extract.

Now, alternate adding the flour mixture and the room temperature milk to the butter-egg mixture, beginning and ending with the flour mixture. With the mixer on low speed, add about a third of the flour mixture, mixing until just barely combined. Add about half of the milk, mixing until just combined. Repeat with another third of flour, the remaining half of milk, and finally the last third of flour. Mix only until the ingredients are just combined; do not overmix.

Gently stir the cooled, melted white chocolate into the finished batter using a spatula, folding just until it’s incorporated. Overmixing at this stage can deflate the batter or make the cake tough.

Pour the batter evenly into your prepared cake pans (dividing by two or three depending on how many pans you’re using). A kitchen scale ensures perfectly even layers. Bake at 350°F (175°C). Baking time varies by layer count: allow approximately 30 minutes for two thicker layers, or 20 minutes for three thinner layers. Check for doneness by inserting a wooden toothpick into the center – it should come out clean. Again, avoid over-baking to prevent a dry cake.

Let the cakes cool in their pans on wire racks for 10 minutes before inverting them onto the racks to cool completely. Remove the parchment paper liners. Ensure the layers are absolutely cool before frosting.

While the cakes cool, prepare the white chocolate buttercream frosting. Start by making the white chocolate ganache component. Break or chop the 4 oz. white chocolate baking bar into small pieces. Combine these pieces with the 1/3 cup of heavy cream in a small microwave-safe bowl. Heat at 50% power in 30-second intervals, stirring well after each, until melted and smooth (again, usually 1 to 1 1/2 minutes total). Set this ganache aside to cool completely to room temperature, which might take about 30 minutes or longer. It should be smooth and fluid but not warm.

In a large bowl (or stand mixer bowl with paddle attachment), beat the softened unsalted butter on medium speed until creamy, about 1 minute. Gradually add the sifted confectioners’ sugar, alternating with the 1/4 cup of additional heavy cream (or milk). Beat on low speed after each addition until just blended. If the mixture seems extremely thick during this process, the recipe notes suggest adding a little of the cooled white chocolate ganache mixture to help it along (though typically liquid is added first).

Once all the sugar and the 1/4 cup liquid are incorporated, add the salt, the completely cooled white chocolate ganache mixture, and the vanilla extract. Beat on low speed until everything is combined. Then, increase the mixer speed to medium and beat for 3 minutes until the frosting is light, fluffy, and smooth. Check consistency; if it’s too stiff, beat in a tiny bit more cream or milk.

Assemble the cooled cake. Place one cake layer on your serving plate. Top with approximately 1 cup of the white chocolate buttercream frosting (or adjust amount based on 2 vs. 3 layers), spreading it evenly. Place the next layer on top and repeat if making a three-layer cake. Place the final layer on top. Use the remaining frosting to cover the top and sides of the cake smoothly with your offset spatula. Decorate as desired – perhaps with white chocolate shavings or sprinkles