1.Spray and line the bottom of a 9″x13″ metal pan with parchment paper. Place egg whites into the bowl of a standing mixer with whisk attachment. Mix on medium speed. When they become a little frothy, add the cream of tartar. Continue on medium speed. Increase to high speed when the volume has doubled. Slowly add in 1/2 cup sugar, while keeping the mixer on. Continue whipping until you see soft peaks that can hold their shape form, about a minute. Place the whipped egg whites into a large mixing bowl, pushing them to one side of the bowl.
2..To the mixer bowl add in flour, 3/4 cup sugar, baking powder, and salt. Whisk until fully incorporated. Add in coconut oil, eggs, guava juice, guava concentrate, and vanilla. Mix on medium low speed for 1 minute until blended, then increase to medium speed for 2 minutes. Preheat oven to 350 degrees.
3.Pour batter into the open side of the bowl with the whipped egg whites, so that the batter flows mostly to the bottom and below the egg whites. Fold the egg whites into the cake batter until the batter is uniform in color, without any streaks of white.
4.Pour batter into the prepared cake pan. Bake for 30-35 minutes, or until toothpick inserted into the center comes out clean. Invert onto a cake rack, leaving cake pan in place to cool completely.
5.In a small saucepan, add cornstarch and 1 cup guava concentrate and whisk together. Cook over medium high heat, whisking frequently until thickened and bubbling. Allow to cook another minute then pour into a shallow bowl and cool to room temperature, about 25 minutes. Cover and chill for about 20 minutes, until it is the consistency of thickened gelatin.
6.In a clean bowl of the standing mixer with whisk attachment add the heavy whipping cream. Whip on medium until barely stiff then add the 1/2 cup guava concentrate and whip until stiff. Set aside.
7.Loosen the cake edges from the sides of the pan. Place a cake platter over the top of the pan and flip everything over, unmolding the cake onto the platter. Peel off the parchment paper from the cake. Use a large serrated knife to split the cake into 2 layers. Flip the top cake over onto a cutting board so that both layers are cut side up. Brush each layer with 1/4 cup of guava concentrate.
8.Place one third of the frosting onto the bottom cake layer and spread the frosting evenly out to the edges. Place the top layer, cut side down, onto the frosted bottom layer. Place some frosting onto the cake top, then push it over the edge and quickly frost the side. Repeat until all the sides are covered. Frost the top last. Cover and chill for a couple of hours or more.
9.After the glaze has chilled, stir it up a bit until it is smooth. Use a rubber spatula to scoop up the glaze to stream it across the cake top in even lines. Repeat until the surface is covered almost to the edges. Use an off set spatula to smooth the lines of glaze together, and gently neaten things up. Cover and chill at least 48 hours before serving.