Preheat the oven to 350 degrees F. Coat the inside of a 9-inch x 9-inch baking pan with non-stick cooking spray. Set aside.
Prepare the vanilla cake mix according to the package directions. Pour into the prepared pan. Bake for 25 minutes or until a toothpick inserted in the center comes out clean.
Remove from the oven and cool completely in the pan on a wire rack. Once cooled, remove from the pan. Cut the cake in half horizontally, then cut into small cubes. It's easiest to cut the cubes while the cake halves are still stacked.
Raspberry Sauce
While the cake bakes, prepare the raspberry sauce by adding the frozen raspberries, powdered sugar, cornstarch, and salt to a small saucepan. Stir to combine.
Place the saucepan over medium-high heat. Bring to a boil while stirring constantly, then continue to cook for 2-3 minutes until thickened. Remove from the heat and cool completely. (You can spread the sauce out over a baking sheet to help it cool faster.)
White Chocolate Cream
Add the heavy cream to a small saucepan and place over medium-low heat. Warm the cream until it reaches 100 degrees F. (Do not bring to a boil.) Put the white chocolate into a medium mixing bowl and pour the hot cream over the top. Let sit for 1-2 minutes and then whisk until smooth. Let cool completely.
Use a hand mixer or a stand mixer to beat the white chocolate cream on medium speed until stiff peaks form. (Don't overmix.)
Whisk together the instant pudding mix and milk in a large mixing bowl for 2 minutes until thickened. Add the whipped cream to the pudding and fold until evenly combined and no longer streaky.
Assembly
Arrange half of the cake cubes in the bottom of a trifle bowl. Spread half of the raspberry sauce over the cake, then top that with half of the white chocolate cream. Arrange most of the fresh raspberries over the cream. Layer the remaining cake, white chocolate cream, and finally the remaining raspberry sauce to the bowl.
Top the trifle with dollops of whipped cream, some fresh raspberries, and white chocolate shavings if desired. Refrigerate until ready to serve.
Notes
You can prepare the raspberry sauce and white chocolate cream to save time on prep if desired. Cover and store in the fridge until you're ready to assemble the trifle. I like to bake the cake and make dinner while the cake cools. Then I can quickly layer everything in the trifle bowl after dinner.
Make individual portions by using short glasses. Divide the trifle elements between the individual bowls so you can get 2 layers in each glass.