Bake the Cake: Prepare the cake mix according to the package directions. Bake in a 9x13-inch pan and let it cool completely. This is important—warm cake will melt the whipped cream!
Make the Whipped Cream: While the cake is cooling, add the cold heavy whipping cream and powdered sugar to a large mixing bowl or the bowl of a stand mixer. Using a hand mixer or stand mixer with the whisk attachment, beat on medium-high speed for 3-5 minutes until you get fluffy, stiff peaks. The whipped cream should hold its shape when you lift the beaters. Homemade whipped cream is key for the best flavor, but you can substitute 8 ounces of Cool Whip if needed.
Prepare the Strawberries: Hull and slice the fresh strawberries into thin slices. Set aside.
Cut the Cake: Once the cake is completely cooled, cut it into small 1-inch cubes or squares. You don't need to be too precise—rustic pieces work great!
Assemble the Trifle: In a large trifle dish, clear glass bowl, or individual serving glasses, start layering. Begin with a layer of whipped cream on the bottom, then add a layer of cake cubes, followed by a layer of sliced strawberries. Repeat the layers (whipped cream, cake, strawberries) until your dish is full, ending with a final layer of whipped cream and a few strawberry slices on top for garnish.
Chill and Serve: Cover the trifle and refrigerate for at least 2 hours (or up to overnight) before serving. This allows the flavors to meld together and the cake to soften slightly. Serve chilled and enjoy!