Preheat the oven to 350 degrees. Line a 10 inch round cake pan with parchment paper, butter and flour.

In a mixing bowl, whip together the egg yolks and sugar until pale yellow. Add the orange zest and vanilla, whip until light and fluffy and set aside.

In a small bowl, combine 1 cup of the Brazil nuts with the flour and set aside. Reserve the remaining nuts for the garnish.

In another bowl, beat the egg whites until foamy. Sprinkle in the salt and continue beating until soft peaks form. Alternate folding in the nut and flour mixture, and the beaten yolk mixture, until combined. Pour into the prepared pan.

Bake 25 to 30 minutes, or until lightly browned. Set on a rack to cool, about 10 minutes. Run a knife along the edge to loosen and invert onto a platter. Remove the parchment let cool completely.

Meanwhile, preheat the oven to 300 degrees. Place cake on a baking sheet lined with parchment paper.

Working over a bowl to catch the juices, peel the grapefruits and oranges and cut between the membranes to remove the sections. Remove the seeds. Arrange the sections over the cake. Pour the juice through a strainer and drizzle over the cake.

In mixing bowl, whisk the egg whites until foamy. Gradually add the sugar, whisking until stiff peaks form, about 10 minutes. Gently fold in the reserved 1 cup of ground Brazil nuts.

Spread the meringue evenly over the cake and bake 1/2 hour. Cool on a rack and serve.