CAKE: Line a 9x13-inch metal pan with parchment paper, leaving an overhang. Parchment paper is important for easy removal of the cake. Lightly coat the parchment paper with nonstick spray. If you aren't using parchment paper, grease the pan with nonstick spray. Preheat the oven to 350 degrees F. In a large bowl, whisk 3 eggs with a fork. Add in the 3/4 cup sour cream, 3/4 cup oil, and 1 tablespoon vanilla extract. Stir until completely combined. Add in the cake mix, vanilla pudding mix, and coarsely chopped 1 cup white chocolate chips. Stir well. Add in the 1/2 cup buttermilk (or water) and mix until just combined. Pour the batter into the prepared pan and bake according to package directions for a 9x13-inch pan. Test that the cake is finished by inserting a toothpick into the center; if it comes out dry or with moist crumbs, the cake is finished. Allow cake to cool in the pan, preferably overnight. (Cover the cooled cake with plastic wrap and let it sit out overnight.) Once ready to use the trifle, cut the cake into 1-inch cubes and use a metal spatula to remove cubes from the pan.
STRAWBERRIES: Remove the tops and stems of the strawberries, slice thinly, and add to a large bowl. Top with the 1/2 cup sugar (less if you're using sweet strawberries) and stir gently to combine. Let stand at room temperature for 30 minutes, tossing at least once or twice more. Drain off any accumulated liquid before adding the strawberries to the trifle.
CREAM: Add the 1 cup heavy cream, 2 tablespoons powdered sugar, and 1 teaspoon vanilla (paste or extract) to a stand mixer fitted with the whisk attachment. Whisk on low speed to combine. Gradually increase to high speed until medium-to-stiff peaks form, about 2-4 minutes. Watch carefully to avoid over-beating. In a separate large bowl, mix together the 1 can sweetened condensed milk and 1/2 cup of 1% milk. With a hand mixer, mix in the pudding mix until combined. With a spatula, gently fold the whipped cream into the condensed milk and pudding mixture. Fold until smooth and combined.
ASSEMBLE: Place about 1/3 of cake pieces in the bottom of the trifle dish. Cover with 1/3 of the cream and then 1 layer of 1/3 of the strawberries. Repeat the layers: cake, cream, strawberries. Repeat the layers one last time: cake, cream, strawberries. Serve immediately. If desired garnish with fresh mint.