Place guava nectar concentrate into a small saucepan, and add the cornstarch.
Whisk cornstarch into the nectar concentrate until there are no lumps.
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.
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 as even lines as possible.
Repeat until the surface is covered in a rectangle almost to the edges.
Not too pretty, but we can fix that.
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.
Oops, got to close up that little hole.
You’ll want to smooth the glaze out swiftly, while it’s still pretty soft.
Ok, looks like a pretty neat rectangle.
Beautiful!
Cover and chill at least 48 hours before serving.
