The sponge cake can be prepared ahead of time or while zefir is setting. Preheat oven to 350F; line 2, 8-inch cake rounds with parchment paper. Place the eggs, sugar and vanilla into stand mixer bowl. Whisk on high speed for 5 to 7 minutes, or until the egg mixture is voluminous, white and ribbons off of whisk attachment. In a separate bowl, combine the flour and baking powder. Sift the dry ingredients into the eggs in small amounts, folding gently but thoroughly after each addition.
Divide the batter evenly between the two pans. Bake in preheated oven for 18 to 20 minutes or until golden brown on top. Cool the sponge cake layers completely on wire rack. If making the layers ahead of time, make sure to keep them covered with a clean kitchen towel to avoid drying.
Prepare the zefir/marshmallow layers. Prepare the pans first. Line 2, 8-inch rounds with plastic wrap. Also line a few baking sheets with plastic wrap; set aside. Combine 1/4 cup water with the strawberry jello powder. Stir to combine, then microwave in 15 second intervals until the gelatin is completely dissolved; set aside.
Pour the remaining 1/2 cup water into a small saucepan and add the agar gelatin. Heat the mixture over medium heat for about 3 minutes, until the mixture starts to thicken and resembles apple sauce. Gradually add in the 2 cups sugar, stirring until it’s all combined. Keep cooking the mixture over medium heat until it comes to a simmer. Simmer, not stirring, for 3 minutes. Meanwhile, place the egg whites into stand mixer bowl and whisk on medium speed to soft peak stage.
Once sugar and agar syrup is ready, slowly pour it into the egg whites, whisking at the same time. Whisk for about 1 minute, then add the dissolved strawberry jello, lemon juice and a few drops of food coloring. Turn mixer up to high speed and whisk for about 5 to 8 minutes; the meringue will start to cool down, begin to thicken and pull away from the sides of the mixing bowl.
Work quickly as the marshmallow sets fast. Spread the meringue into the prepared 8-inch cake rounds, creating 1-inch thick layers. Use an off-set spatula to level off the top. Transfer the remaining meringue into a pastry bag (I used tip #847) and pipe onto prepared baking sheets; the marshmallow size is up to you. Sprinkle the tops of the marshmallows/zefir with confectioner’s sugar. Set it aside for at least 1 hour, or overnight, to set. The longer it sets, the crispier the outside will become.
Prepare the buttercream. Place softened butter into a mixer bowl and whisk until light and fluffy. Add the confectioner’s sugar and strawberry emulsion (can also use extract with a few drops of red food coloring). Whisk on high speed until light and fluffy.
Prepare the strawberry filling. Place strawberries into a large bowl and crush using a masher or a large fork.
Assembling the cake: split the cooled sponge cake layers in half, creating 4 layers total. Spread strawberry puree onto the first layer. Remove the zefir layer from the pan and invert on the sponge cake layer, removing the plastic wrap. Spread with butter cream and continue assembling the cake, alternating with sponge cake, strawberries and zefir. Cover the outside of the cake with butter cream and smooth down sides with a flat spatula. Set the cake into the refrigerator to set for 30 minutes.
Prepare the chocolate ganache. Heat the heavy cream until steaming hot (not boiling!). Add the chocolate melts/chips and stir until the ganache is smooth. Allow it to cool slightly before pouring over the top of the cake.
Garnish the top of the cake with fresh strawberries and zefir dipped in chocolate. Keep any remaining zefir covered in an airtight container; it can stand for up to 1 week, no refrigeration needed. Refrigerate cake; remove 1 hour prior to serving to allow the butter cream to soften.