Begin by preheating your oven to 350 degrees and greasing a 13x9 pan with cooking spray. Pour 3 cups of crushed strawberries into the prepared pan, making sure they are spread in an even layer across the bottom. This will become the beautiful, caramelized fruit topping once the cake is inverted.
Next, sprinkle the entire 6-ounce package of strawberry Jell-O gelatin dessert evenly over the crushed strawberries. The gelatin will help bind the fruit and add extra strawberry flavor throughout.
Top this layer with 4 cups of mini marshmallows arranged in a single layer. These marshmallows will create pockets of gooey sweetness and help support the cake batter as it bakes.
In a mixing bowl, combine the cake mix, 1 cup of water, 1/4 cup of canola oil, and 3 egg whites. Mix these ingredients together until completely incorporated, being cautious not to overmix as this can result in a dense cake.
Pour this batter directly over the marshmallow layer, spreading it evenly across the top. The batter will sink slightly and partially mix with the layers below, which is exactly what you want for this dessert.
Place the pan in the preheated oven and bake for 35 to 40 minutes, or until a knife inserted into the middle of the cake comes out clean. This guarantees the cake is fully cooked while the fruit and marshmallows below remain moist and gooey. Allow the cake to cool completely in the pan before carefully running a knife around all edges and inverting it onto a serving platter. For convenience, many people prefer to serve it directly from the 13x9 pan instead.
