Preheat your oven according to the cake mix box directions. Prepare a 9x13 inch baking dish with cooking spray or parchment paper. Combine the white cake mix with 1.25 cups water in a large bowl and mix until just combined—don't overmix, as this keeps the cake tender. Pour the batter into the prepared dish and bake according to box directions until a toothpick inserted in the center comes out clean. Remove from oven and let cool for about 15 minutes before proceeding to the next step.
While the cake cools slightly, prepare the strawberry gelatin by dissolving it in 1 cup of boiling water, stirring for about 2 minutes until completely dissolved. Add 1/2 cup cold water to the gelatin mixture and stir well. Once the cake has cooled for about 15 minutes, use a fork or spoon handle to poke holes all over the surface of the cake—aim for dense coverage so the gelatin can seep deeply into the cake and flavor every bite. I like to poke holes in a crosshatch pattern, which ensures even distribution of the jello throughout the cake.
Carefully pour the cooled gelatin mixture over the entire surface of the cake, pouring slowly so it can absorb into the holes you've created rather than pooling on top. Make sure to distribute it evenly across all areas. Place the cake in the refrigerator for 3-4 hours, allowing the gelatin to set completely and fully hydrate the cake.
About 30 minutes before you're ready to assemble, remove the cold milk directly from the refrigerator. In a medium bowl, whisk together the instant vanilla pudding mix with the cold milk, stirring constantly for about 2 minutes until the pudding reaches a thick, spreadable consistency. Stir in the pure vanilla extract and a pinch of salt—the salt enhances the vanilla flavor and adds depth to the pudding layer. I find that using pudding straight from the mix rather than the pre-made version gives you better control over the texture and allows you to customize the vanilla flavor.
Remove the cake from the refrigerator and spread the vanilla pudding mixture from Step 4 evenly over the entire top using a spatula, working gently so you don't disturb the set gelatin layer beneath. Top the pudding with the thawed whipped topping, spreading it in a smooth, even layer that covers the pudding completely. Chill the finished cake for at least 1-2 hours before serving so all the layers set together, creating clean slices with distinct layers of cake, gelatin, pudding, and whipped topping.