Your guests will learn appearances can be delightfully deceiving as they dive into the ladylike layers of this charming cake. It's actually meringue, split and filled with strawberry-rhubarb compote and cream whipped with creme fraiche.
Preheat oven to 225 degrees. Line every other cup of two standard muffin tins with paper liners; coat liners with cooking spray. With a mixer on medium-high speed, use whisk attachment to whisk egg whites, vinegar, vanilla, and salt until frothy. Add sugar, 1 tablespoon at a time, beating 2 minutes after each addition and scraping bowl to make sure all sugar dissolves. Continue whisking until firm, glossy peaks form.
Transfer meringue to a pastry bag fitted with a large plain tip, and pipe into lined cups so meringue is about 2 inches above rims. Bake until meringues are just beginning to color and are completely dry on the outside but still soft in the middle (a toothpick inserted at base of top, through the side, should have moist but cooked crumbs attached), 3 to 3 1/4 hours, rotating tins halfway through. Remove from oven, immediately remove from tins, and peel off liners. Transfer to rack to cool completely.
Just before serving, whisk together creme fraiche and heavy cream until soft peaks form. Using a serrated knife, gently slice tops off cupcakes (don't worry if they crack). Spoon compote and then cream onto bottom halves, and replace tops. Serve immediately.