Spray the bottom of a 9×13 pan with cooking spray.: The pan will smell faintly of oil as you coat it, which helps prevent sticking and ensures the bottom layer can be removed cleanly if you lift it out. Greasing is important because the softened graham crackers can cling to bare metal; a light, even spray suffices. A common mistake is over spraying which pools oil in corners, so wipe away excess with a paper towel if needed to avoid greasy spots.
Line the bottom of the pan with whole graham crackers. Layer with half of the strawberry slices.: When you press the whole graham crackers into the pan, you will hear a gentle crackle and feel the rigid texture under your fingers. Arrange them edge to edge for an even base, then scatter half the sliced strawberries so their red slices peek through the cracks. The fruit releases a bit of juice which mingles with the crackers, initiating the softening process. Avoid overlapping the crackers too much, which can create thick, uneven spots that do not soften consistently.
In the bowl of an electric mixer, combine pudding mix and milk; beat at medium speed for 2 minutes. Fold in whipped topping.: As you beat the vanilla pudding mix with cold milk , you will hear the mixer hum and see the mixture thicken and turn glossy. Two minutes usually gives a silky, scoopable texture. When you fold in the thawed Cool Whip , use a rubber spatula and gentle turns so the air stays trapped, making the filling light. Over mixing here will deflate it, producing a denser result, so fold until streaks disappear but stop when the mixture looks uniformly pale and airy.
Pour half the pudding mixture over strawberries. Place another layer of whole graham crackers on top of the pudding layer. Top the crackers with remaining strawberry slices.: Pouring the pudding will create a thick, creamy blanket that mutes the sharpness of the strawberries , and you will see the filling settle into gaps around the crackers. Smooth it lightly with a spatula so it reaches the edges. Then place another even layer of whole graham crackers . Their surface will glisten slightly as they absorb moisture, so work quickly to keep layers neat. Arrange the remaining strawberries in a single layer so each bite will have fruit; a common pitfall is piling berries which can create juice pockets and uneven texture.
Pour the remaining half of the pudding mixture over the strawberries and cover with another layer of graham crackers.: The second half of the filling brings everything together, sinking around the fruit and creating cohesion between layers. As you smooth the top, listen for a soft sloshing as it finds its level, then place the final layer of whole graham crackers . Make sure the top crackers are aligned for a tidy look. If the filling appears too thin at this point, gentle pressing with a spatula will help it compact; avoid pressing so hard that you force the layers to separate.
Heat the container of prepared frosting, uncovered in the microwave for 30 to 45 seconds. Pour over the top of the cake.: Warming the vanilla frosting transforms its thick spreadable texture into a glossy, pourable glaze. You will notice a warm vanilla scent as the frosting loosens. Pour it slowly in a ribbon motion to cover the entire surface, then tilt the pan slightly or use a spatula to spread it to the edges. Heat only until pourable, overheating can thin the frosting too much and it will not set properly, causing the top to run off the sides.
Refrigerate for at least 8 hours before serving. Top with additional strawberry slices if desired.: Chilling is where the magic happens, the layers knit and the graham crackers soften into a cake like texture. You will notice a firmer slice after refrigeration, with the pudding set and the frosting slightly firm to the touch. Resist the urge to cut it too soon, otherwise the slices will be sloppy and the texture not fully developed. If you must serve earlier, keep slices on the shorter side and expect a softer, less cohesive presentation.
Top with additional strawberry slices if desired.: Right before serving, arrange extra strawberries on the frosted top for color and freshness. Their bright aroma will lift the scent of the whole dish and the contrast between glossy frosting and fresh fruit is visually appealing. Add them just prior to serving to avoid releasing too much juice onto the frosting which can streak the top and make it look wet.