In a medium-sized saucepan, combine the frozen raspberries, sugar, and lemon juice. Set the pan over medium heat, stirring frequently as the mixture comes to a gentle boil. Continue cooking for about 3-4 minutes, being cautious to prevent the raspberries from sticking or burning.

In a small bowl, mix the cornstarch with water until it dissolves completely, forming a slurry. Gradually whisk the slurry into the raspberry mixture while constantly stirring. Cook for another minute or two until the mixture thickens. Remove from the heat and allow it to cool fully. As the filling cools, it will further thicken. Once at room temperature, transfer the raspberry filling to a bowl, cover it with plastic wrap, and refrigerate for at least 2 hours or until ready for use. Ensure the filling is thoroughly chilled before assembling the cake.

Preheat your oven to 350°F. Prepare three 8-inch round cake pans by spraying them with non-stick baking spray.

In a large mixing bowl, combine the white cake mix, white chocolate pudding mix, egg whites, milk, canola oil, and vanilla extract. Using an electric hand mixer, beat on medium speed for 2 minutes, ensuring the batter is smooth and lump-free.

Evenly distribute the thick batter among the prepared cake pans, using an offset spatula to spread it into a uniform layer. For accuracy, use a kitchen scale to divide the batter equally, about 1 cup per pan. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Allow the cakes to rest in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Melt the white chocolate by placing it in a microwave-safe bowl and heating in 30-second intervals, stirring between each interval until smooth. Set aside to cool to room temperature.

In the bowl of a stand mixer fitted with a paddle attachment, beat the unsalted butter and salt on medium-high speed for 1-2 minutes, until fluffy and pale. Gradually add the sifted powdered sugar, one cup at a time, starting on low speed and increasing the speed to prevent the sugar from scattering. Pause occasionally to scrape down the sides of the bowl, ensuring everything is evenly mixed.

Once all the powdered sugar has been incorporated, slowly pour in the cooled melted white chocolate while continuing to beat the frosting. Add the heavy cream and vanilla extract, and beat for another 2-3 minutes, until the frosting is smooth, light, and airy. Set aside 1½ cups of frosting in a piping bag fitted with a tip (or use a zip-top bag with the corner snipped off) to use later for garnishing and piping around the cake layers.

If the tops of your cake layers are slightly domed, trim them with a serrated knife to create flat, even layers. Start by placing the first cake layer on your serving platter or cake stand. Spread a thin layer of white chocolate frosting over the top, then use the reserved frosting in the piping bag to pipe a circle around the edge of the cake. This creates a barrier to keep the raspberry filling contained.

Spread half of the chilled raspberry filling inside the piped circle, ensuring it doesn’t overflow. Place the cake in the freezer for 10-15 minutes to set the filling. Repeat this process for the second cake layer, adding frosting and raspberry filling, and freeze once more for 10-15 minutes.

Finally, place the third cake layer on top. Use the remaining white chocolate frosting to cover the top and sides of the cake, smoothing it out for an even finish. Pipe a decorative border around the top edge of the cake with the frosting in the piping bag. Garnish the top with fresh raspberries arranged in a circle within the border, and sprinkle the center with white chocolate shavings for an elegant touch.