Yield: makes 12- 3" entremet rounds
In the bowl of a stand mixer fitted with a paddle attachment, cream together butter, sugar and salt. Mix until smooth. Add the yolk and continue to mix on medium speed until incorporated. Add all the flour and mix on low until the dough is crumbly and moist- about 45 seconds.
Dump dough onto a lightly floured surface and press dough together into a cohesive mass. Press down until dough is about 1/2 inch thick. Wrap tightly in plastic wrap and let chill in the refrigerator for at least 30 minutes until cold.
Preheat oven to 350 degrees F. Place chilled dough on a lightly floured surface. Divide dough in half. Roll out one half of dough into a even layer of about 1/4'' thick. Place an 8-inch cake ring over the dough. Trace a circle around the outside of the cake ring.
Gently lift dough and place it on a parchment lined baking sheet. Use a dough docker or a fork to dock/punch holes all over the dough. Bake in a preheated oven until dough is a nice golden brown- about 20-30 minutes. Let crust cool in pan for a couple of minutes before transferring to wire rack to cool to room temperature. (Save remaining half of dough for other recipes. Dough can be kept in fridge for up to 3 days or frozen for up to two weeks.)
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.
Combine yogurt and sugar in small bowl.
Warm up puree. About 15-20 seconds in the microwave. Add bloomed gelatin (squeezed of excess water) into warmed puree. Stir until gelatin is dissolved.
Add yogurt mixture into the puree. Fold in whipped cream. Pour into prepared molds immediately. The cold whipped cream will set the gelatin fairly quickly. Let sit on counter or place in fridge until ready to pour mirror glaze.
Add water, puree and sugar to sauce pot. Heat and stir until sugar is dissolved. Remove from heat and add bloomed gelatin. Stir until gelatin is dissolved. Let sit at room temperature until slightly warm to the touch. Pour over prepared and set yogurt mousse. The mirror glaze to set instantly because of the cold mousse.
Line 3″ entremet rounds with acetate strips. Place shortbread cookie bottom inside. Spread a little apricot jam to allow white sponge cake to adhere. Follow with yogurt mousse, then finish with mirror glaze. Let chill in the fridge to set up. Unmold cake from entremet rounds and remove acetate strip before serving.