Step 1: Prepare the Cake

1. Preheat the oven: Set it to 350°F (175°C) and grease and flour two 9-inch round cake pans.
2. Combine dry ingredients: In a medium bowl, sift together the all-purpose flour, baking powder, baking soda, and salt. This aerates the flour and ensures an even distribution of the leavening agents.
3. Cream butter and sugar: In a large mixing bowl, cream the softened butter and granulated sugar until light and fluffy, about 3-5 minutes. This process incorporates air into the mixture, which is essential for a light cake.
4. Add eggs and vanilla: Add eggs one at a time, beating well after each addition until fully incorporated. Mix in the vanilla extract.
5. Melt the white chocolate: Use a heatproof bowl over simmering water or microwave it in short bursts, stirring until smooth. Allow it to cool slightly before adding to the batter.
6. Combine wet and dry ingredients: Alternate adding the dry ingredients and buttermilk to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined to avoid overmixing.
7. Divide and bake: Pour the batter evenly into the prepared cake pans, smoothing the tops with a spatula. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
8. Cool the cakes: Allow the cakes to cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Step 2: Prepare the Raspberry Filling

1. Cook the filling: In a medium saucepan, combine fresh raspberries, sugar, cornstarch, and lemon juice. Cook over medium heat, stirring frequently until the mixture thickens and begins to boil. Allow it to boil for 1 minute while continuing to stir.
2. Cool the filling: Remove from heat and let it cool completely before using. This ensures it will set properly when layered in the cake.

Step 3: Prepare the White Chocolate Frosting

1. Beat the butter: In a large bowl, beat the softened butter until smooth and creamy.
2. Incorporate powdered sugar: Gradually add the powdered sugar, beating well after each addition until the mixture is fluffy.
3. Add chocolate and cream: Mix in the melted white chocolate, heavy cream, and vanilla extract. Beat until the frosting is light and fluffy, ensuring even distribution of the chocolate.

Step 4: Assemble the Cake

1. Layer the cake: Place one cake layer on a serving plate and spread a generous amount of raspberry filling over the top, allowing some to drip down the sides for a rustic look.
2. Add the second layer: Place the second cake layer on top and cover the entire cake with white chocolate frosting, using a spatula to smooth the surface and sides.
3. Garnish: Decorate with fresh raspberries and white chocolate shavings for an elegant finish that will impress your guests.