Preheat oven to 325°F. Line baking sheet with parchment paper.

In bowl, toss together rhubarb, cane sugar, vinegar and a few spoonfuls of water; transfer to pot. Cook over medium heat until rhubarb is tender. Remove from heat and let cool.

In bowl of stand mixer with whisk attachment, beat egg whites and sugar until glossy and stiff. Using rubber spatula, fold in coconut flour and chocolate. Spread meringue into desired shape on prepared baking sheet, and bake for about 25 minutes.

In clean bowl of stand mixer with whisk attachment, beat whipping cream until medium peaks form; gently fold in skyr. Split vanilla beans in half lengthwise and scrape out seeds; rub into sugar, then fold into cream mixture.

Spread cream mixture over meringue base, layer with rhubarb and top with pistachios and more chocolate if desired.