Originally 'Relgieuses' the french choux pastry were supposed to depict a nun on her knees praying, the bun would be her praying from behind, the icing being her habit, her head hidden between her knees... The story has been lost along the years, hence why bakers have added the 'head' of the nun, the upper bun.
I filled these with Creme Patissiere, a french custard. Yummy! Let me know if you'd like the recipe!