Preheat oven to 350 degrees. Line bottom of a 9″x13″ metal pan with parchment paper. Trim edges to fit bottom.
Place egg whites into the bowl of a standing mixer. 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 the ½ c. sugar, while keeping the mixer on. Continue whipping until soft peaks that can hold their shape form (this should only take a minute or so). Place the whipped egg whites into a large mixing bowl, pushing them to one side of the bowl.
In a separate mixing bowl, whisk together flour, ¾ c. sugar, baking powder, and salt. Transfer it into the standing mixer bowl. Add in oil, eggs, water, nectar concentrate, and vanilla. Mix on medium low speed for 1 minute until blended, then increase to medium speed for 2 minutes.
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. Place a spatula down one side of the bowl to the bottom, push batter from underneath to the other side of the bowl, then lift spatula up the other side of the bowl and over the top, carrying some thick batter with it. Continue this pattern, repeating it while turning the bowl ¼ turn at a time, and seeing the streaks of cake batter and egg whites slowly incorporate together.
The final chiffon cake batter should be uniform in color, without any streaks of white (those make tough areas in the cake). If necessary, use the spatula to gently spread through the white streaks to help blend them into the chiffon cake batter. Whatever you do, do not stick the spatula into the bowl and stir vigorously in circles–that would deflate the air bubbles and yield a dense cake.
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, and cool completely.
Make sure the ½ c. guava nectar concentrate is thawed.
Whip the cream in an electric mixer until barely stiff. Add the ½ c. guava nectar concentrate and whip until stiff.
Guava Glaze (make after the frosted cake has chilled a few hours)
In a small saucepan, whisk cornstarch into the nectar concentrate. Cook over medium high heat, whisking frequently until thickened and bubbling. Allow to cook another minute. Pour into a shallow bowl and cool to room temperature (~20-25 minutes). Cover with plastic wrap and chill about 20 minutes, until it is the consistency of thickened gelatin—no firmer, or it will be difficult to glaze over the whipped cream frosting.
Filling and Frosting the Cake
Loosen the cake edges from the sides of the pan with a butter knife. 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. Insert a flexible cutting mat between the 2 layers to lift off the top layer. Invert this top layer onto another mat so that both layers are cut side up. Brush each layer with half of the guava syrup.
Place one third of the frosting onto the bottom cake layer. Use a small, metal angled spatula to spread frosting evenly out to the edges. Place the top layer, cut side down, onto the frosted bottom layer.
Working a small section at a time, 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 to set the frosting.
Topping the Cake
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 in a rectangle almost to the edges. Use the flat side of the spatula or a small, angled frosting spatula (better) to smooth the lines of glaze together, and gently neaten things up. Try not to flick up any whipped cream into the glaze.