Making this strawberry poke cake involves three distinct components that come together beautifully. Start by creating the strawberry liquid that will soak into the cake. Combine 3 cups frozen strawberries, 1/4 cup sugar, 2 tablespoons orange juice, and 1/2 cup water in a saucepan.
Cover and cook over medium heat until the strawberries break down and become soft—no need to thaw them first. While this mixture cooks, prepare your cake batter by preheating the oven to 350°F and buttering and flouring a 13x9x2 baking pan.
In a large bowl, whisk together 2 1/4 cups all-purpose flour, 1/2 teaspoon salt, and 4 teaspoons baking powder. In a separate bowl, whisk together 1 cup milk, 6 egg whites, and 2 teaspoons vanilla extract until the egg whites are just broken up.
The cake assembly follows a specific mixing order that's pivotal for achieving the proper texture. Using a stand mixer, cream together 1 3/4 cups white sugar and 12 tablespoons room temperature butter for about 3 minutes until fluffy.
Reduce the mixer speed to low and alternate adding the wet and dry ingredients: first half the wet ingredients, then half the dry ingredients, then the remaining wet, followed by the remaining dry. Pour this batter into your prepared pan, spread evenly, and bake on the middle rack for 35 minutes.
While the cake bakes, separate the cooked strawberry solids from the liquid, setting the solids aside. Stir 2 tablespoons strawberry gelatin into the liquid and let it stand until the cake is done.
Once the cake has cooled for at least one hour, the real magic happens. Using a large skewer, poke the cake about 50 times, being careful not to poke all the way to the bottom or the gelatin will run underneath.
Pour the strawberry gelatin mixture all over the top and refrigerate for 3 hours or up to a day. For the final topping, combine the reserved cooked strawberry solids with 1 cup frozen strawberries and 2 tablespoons sugar in a food processor, pulsing until it reaches a jam-like consistency. Spread this mixture evenly over the cake, then finish with a layer of whipped cream.