Preheat oven to 350°F (175°C). Grease a 13×9-inch baking pan
In a bowl, whisk flour and baking powder
In a separate bowl, cream butter and granulated sugar until pale and fluffy
Add egg whites one at a time, mixing well after each addition. Add vanilla extract
Gradually fold in flour mixture alternating with milk, starting and ending with flour
Pour batter into prepared pan and bake 25-30 minutes, until a toothpick inserted in center comes out clean
Meanwhile, beat strawberry jam with 2 tablespoons water in a small saucepan until smooth
Once cake is cooled slightly, spread jam syrup over warm cake
Top with strawberries and refrigerate 15 minutes
In a chilled bowl, whip heavy cream, powdered sugar, and salt until stiff peaks form
Spread whipped cream over strawberries and refrigerate cake at least 1 hour before serving