Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking dish or two 9-inch round cake pans, depending on your preference.

Prepare the cake mix according to the directions on the package. Pour half the batter into the prepared baking dish. To the remaining batter, gently swirl in the strawberry gelatin powder (just the dry powder, do not dissolve it).

Pour the strawberry batter over the vanilla layer and gently swirl them together with a knife to create the marble effect. Bake in the preheated oven for 35–40 minutes, or until a toothpick inserted in the center comes out clean.

Let the cake cool completely in the pan on a wire rack. This will take about 30 minutes. You want the cake to be completely cooled before topping it.

In a medium mixing bowl, combine the instant vanilla pudding mix, cold milk, and vanilla extract. Whisk until the mixture thickens slightly (about 2 minutes). Set aside.

In a separate bowl, prepare the whipped topping mix according to the package directions (usually involves whipping it with cold milk). Once fluffy, fold it gently into the vanilla pudding mixture until smooth and creamy.

Spread the whipped vanilla topping over the cooled cake evenly using a spatula. Chill the cake in the refrigerator for at least 1 hour before serving to let the flavors meld together and the topping set nicely.