Combine 3 cups strawberries, 1/4 c sugar, orange juice and water in a saucepan. Cover and cook over medium heat until strawberries break down and become soft. (Strawberries do not need to be thawed before beginning.).
Preheat oven to 350°F Butter and flour a 13x9x2 baking pan.
In a large bowl, whisk together flour, salt and baking powder.
Whisk together milk, egg whites and vanilla, just until egg whites are broken up.
In the work bowl of a stand mixer (or using an electric hand beater), cream together 1 3/4 cups sugar and butter, about 3 minutes, until fluffy. Reduce speed to low. Add half the wet ingredients, then half the dry ingredients, then the remaining wet followed by the remaining dry. (This step IS important to get a nice flat cake!).
Pour batter into prepared pan. Spread evenly. Bake on middle rack of preheated oven for 35 minute.
Separate strawberry solids from liquid; set the solids aside. Add gelatin powder to liquid. Stir well and let stand until cake is baked.
When done, set cake on a baking rack and let cool at least one hour. Using a large skewer, poke cake about 50 times. Do not poke all the way to the bottom, or the gelatin will run underneath. Pour gelatin all over the top of the cake. Refrigerate 3 hours, or up to 1 day in advance.
Combine reserved cooked strawberry solids with 1 c frozen strawberries and 2 tbsp sugar in the work bowl of a food processor. Pulse until it has the consistency of strawberry jam. Spread evenly over top of cake. Spread cake with whipped cream.