Preheat the oven to 325°F (163°C) and prepare a 9×13 metal pan or baking dish by spraying the sides with baking spray and lining the bottom with parchment paper.

In a medium bowl, add the flour, baking powder, baking soda and salt. Whisk together and set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter at high speed until creamy, about 2 minutes. Add the sugar and cream it together with the butter at medium-high speed until light and fluffy, about 2 minutes, scraping the bowl and paddle at the halfway point.

Turn the mixer to low and add the egg whites one at a time, mixing until they are just combined and scraping down the bowl and paddle as needed. Add the vegetable oil and vanilla, increase the mixer speed to high and beat for 1 minute.

Turn the mixer off and add the flour mixture all at once. Mix the ingredients on low speed until just combined, then slowly pour in the buttermilk. Continue mixing at low speed for about 30 seconds, until the batter is uniform and combined. The batter will be slightly thin.

Pour the batter into the prepared pan and bake for 30-35 minutes, until a toothpick inserted comes out clean. Cool completely before topping.

Start by hulling and halving the strawberries. Reserve about ⅓ of the strawberries and set them aside. Add the remaining strawberries into a medium saucepan along with the granulated sugar and lemon juice.

Stir the strawberry mixture to combine and cook over medium heat until the strawberries release their juices and the mixture begins to simmer. Reduce the heat slightly and continue to simmer for about 10 minutes.

Meanwhile, stir together the cornstarch and water in a small bowl. Remove the strawberry mixture from the heat and stir in the cornstarch mixture, then continue to cook, stirring frequently, until the strawberry sauce is thick enough to coat the back of your spatula, 1-2 minutes.

Remove the strawberry sauce from the heat and fold in the reserved strawberries, then place in an airtight container in the refrigerator to cool for about an hour, or up to 5 days ahead of assembling the cake.

Place the mixing bowl of your stand mixer into the refrigerator for about 20 minutes to chill before you begin.

Into the cold mixing bowl, add the cream cheese and beat on medium speed with the whisk attachment until creamy, about 2 minutes. Add the powdered sugar and mix on low speed until mostly incorporated, then turn the mixer to medium-high speed and mix for about 1 minute.

Add the vanilla extract, then turn the mixer to medium-high speed and slowly pour in the heavy whipping cream. Once all the heavy whipping cream has been added, continue mixing on medium-high for just about 30 seconds longer, keeping a close eye on the mixing bowl to watch it thicken.

Use the whipped cream cheese frosting immediately to assemble the strawberry shortcake sheet cake, or store it in the refrigerator in an airtight container for up to 3 days ahead of time.

Transfer the vanilla sheet cake to a serving platter. Add all of the whipped cream cheese frosting on top and use an angled spatula to smooth it to the edges.

Finally, top with the strawberry sauce, leaving a little room around the edges so the whipped cream cheese frosting can peek through. Garnish with fresh strawberry halves and slices.