Prepare a 9x13 sheet pan with butter and parchment paper and set aside.
Blend the 4 cups strawberry in a mixer, then simmer in a small sauce pan with 2 tablespoon sugar to reduce the mixture. Stir so it doesn't burn.
Once the mixture is reduced to half (2 cupand is thicker in texture, like a jam, remove and cool in the refrigerator. You will use this later.
Sift together flour, baking powder and salt and set aside.
Beat the butter and sugar on high until light and fluffy
Using a fork whisk egg whites, milk and extract together in a bowl.
Alternate the dry and wet ingredients into the butter mixture beating on medium until incorporated.
Split the batter evenly into two bowls.
Add in 1 cup strawberry preserves... the other cup will be for the frosting so save it.
Stir the preserves until it's fully blended.
Set the batter in the fridge and cool the chocolate cake to make the Neapolitan.
Once all three cakes are finished, pour a line of the vanilla batter in the pan, then chocolate, then strawberry and alternate until the pan is filled.
Bake in a preheated 350 degree oven for 30 minutes.
Do you not over bake or they will become dry!
Don't worry if the layers are not perfectly even, then will cook together still creating a neapolitan look.
Let the cake cool before removing and frosting.