Preheat oven to 350 degrees F. Butter an 8-inch round pan and line with parchment paper. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, combine flour, sugar, baking powder, and salt. Slowly add butter and mix on low-medium speed until crumbly.
In a measuring glass, whisk together egg whites, milk, and vanilla. Add 1/2 of wet mixture to mixing bowl at medium speed. Add remaining half of wet mixture and continue to mix until combined.
Transfer batter to prepared pan. Bake for 20-30 minutes until cake is set and golden in color. Insert a toothpick in center of cake to test doneness. Let sit in pan for a couple of minutes before molding. Let cool to room temperature on wire rack. Slice cake into desired thickness.
Place shortbread crust on a parchment lined sheet tray. Place 8 inch cake ring around crust. Line cake ring with an acetate sheet. If desired, spread a thin layer of strawberry jam over shortbread crust. Gently place cooled cake on top of shortbread crust.