The top of the cake will look very dark and slightly overbaked — this helps keeps the cake from becoming too soggy after the gelatin is poured over the top.1.
For the cake: Adjust an oven rack to the middle position and heat the oven to 350 degrees.
Grease and flour a 13 by 9-inch baking pan.
Whisk the flour, baking powder and salt together in a medium bowl.
In another medium bowl, whisk the milk, vanilla and egg whites together.2.
In a large bowl, beat the butter and sugar together with an electric mixer on medium-high speed until light and fluffy, 3 to 6 minutes.
Reduce the mixer speed to low and beat in one-third of the flour mixture, followed by half of the milk mixture.
Repeat with half of the remaining flour mixture and the remaining milk mixture.
Beat in the remaining flour mixture until just incorporated.3.
Give the batter a final stir with a rubber spatula to make sure it is thoroughly combined.
Scrape the batter into the prepared pan, smooth the top, and gently tap the pan on the counter to settle the batter.
Bake the cake until a toothpick inserted in the center comes out clean and the top is very brown, about 35 minutes, rotating the pan halfway through.
Cool the cake completely in the pan on a wire rack, about 2 hours.4.
For the syrup and topping: Combine 3 cups of the strawberries, 2 tablespoons of the sugar, water and orange juice in a medium saucepan, cover and cook over medium-low heat until the strawberries are softened, about 10 minutes.
Strain the mixture into a medium bowl, reserving the strained solids.
Whisk the gelatin into the liquid and cool to room temperature, about 20 minutes.5.
Use a wooden skewer to poke 50 large holes in the cooled cake.
Don't poke the cake through to the bottom, but twist the skewer when poking to enlarge the holes.
Pour the cooled gelatin mixture evenly over the top of the cake, making sure to cover the holes.
Cover the cake with plastic wrap and refrigerate until the gelatin is set, about 3 hours.6.
Pulse the reserved strained strawberries, 2 more tablespoons sugar, and the remaining 1 cup frozen strawberries together in a food processor until the mixture resembles strawberry jam, 5 to 7 pulses.
Spread the mixture evenly over the cake.
(The cake can be covered with plastic wrap and refrigerated for up to 3 days.)7.
Before serving, with an electric mixer on medium-high speed, whip the cream and the remaining 2 tablespoons sugar together to soft peaks and spread evenly over the cake.