Preheat oven to 350° 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 ½ 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 8 inch cake ring (2-inch high) on serving plate. Line cake ring with an acetate strip (2.5inch high strip). Place cooled cake inside cake ring.
Strawberry Mousse:
In a medium bowl, whisk together yogurt and sugar. Set aside.
Fill a small bowl with ice cold water to bloom gelatin sheet for 3-5 minutes. Squeeze out excess water from gelatin. Place soft gelatin sheets into strawberry puree. Microwave mixture 10 seconds at a time until gelatin has melted. Transfer to a large bowl and let cool slightly.*If using gelatin powder: stir gelatin powder into strawberry puree. Let sit for 5-7 minutes until gelatin powder has absorbed moisture. Then, microwave mixture 10 seconds at a time until gelatin mixture is melted and smooth.
Add sugar yogurt mixture to slightly cooled strawberry puree. Mix until combined.
In another bowl, whip heavy cream to soft peaks. Add whipped cream to the strawberry yogurt mixture in three additions. Gently fold cream into mixture.
Arrange sliced strawberries around the perimeter of cake ring on top of white cake. Pour strawberry yogurt mousse over white cake into cake ring. Use a mini offset spatula to spread mousse into an even layer. Place in refrigerator to chill for 15-30 minutes(or longer) while you prepare jelly glaze.
Strawberry Jelly Glaze:
In a bowl of ice cold water, bloom gelatin sheet for 3-5 minutes. Squeeze out excess moisture.*If using gelatin powder: Stir gelatin powder in ¼ cup (2 oz) cool water. Let sit for 5 minutes until gelatin has absorbed liquid.
Place bloomed sheet gelatin in a small bowl of 2 oz water with strawberry puree and sugar. Microwave for 10 seconds at a time until gelatin has melted. Whisk to ensure smooth consistency. Let cool to room temperature.*If using gelatin powder: Stir gelatin mixture with strawberry puree and sugar. Microwave for 10 second intervals until gelatin mixture has melted. Whisk to ensure smooth consistency. Cool to room temperature.
Take out chilled mousse cake and pour room temperature jelly glaze over cake. Use a torch to remove any bubbles or foam that may form. Return to fridge to let jelly glaze set. Keep in fridge until ready to serve.
Notes
Over-mixing the whipped cream will result into a grainy textured mousse. Whip whipped cream to SOFT peaks. It will continue to stiffen as you fold soft whipped cream into other ingredients.
Use plain nonfat or low-fat yogurt.
Strawberry puree will contain bits of seeds. Strain out if desired.
Storage and Leftovers:
Keep acetate strip on until you are ready to serve cake. Otherwise, you run the risk of drying out the edges of the cake.
Uncut, fully assembled cake will keep in the fridge for up to 3 days. Overtime, the mousse will start to separate.
Store cake slices in an airtight container for up to 3 days.