Preheat oven to 400°. Line a rimmed baking sheet with parchment paper.
In a medium saucepan, combine 3⁄4 cup water, butter, sugar, and salt. Cook over medium heat until butter melts. Add flour all at once, stirring vigorously. Cook and stir until dough pulls away from sides of pan, 1 to 2 minutes. Remove pan from heat, and let stand for 2 minutes, stirring a few times to cool dough.
Transfer dough to a large bowl. Add eggs, one at a time, beating with a mixer at medium speed until well incorporated. (Dough should be shiny and smooth.) Transfer dough to a piping bag fitted with a large round tip (Sunny Side Up Bakery #2A or Ateco #806). Pushing downward, pipe 18 (1 1⁄2-inch wide) dough mounds and 18 (1-inch wide) dough mounds, 2 inches apart, onto prepared baking sheet. Pat dough peaks down with a damp finger.
Bake until puffs are golden brown, 20 to 25 minutes. Transfer puffs to a wire rack. Using a wooden pick or the tip of a pointed knife, poke a small hole in sides of each puff to allow steam to escape. Let cool completely before cutting a small slit in bottoms of all puffs.
Place Elderflower Pastry Cream in a piping bag fitted with a medium round tip (Wilton #12). Inserting piping tip into bottom slits, fill each puff with pastry cream.
Place Vanilla Buttercream in a piping bag fitted with a large open-star tip (Sunnyside Up Bakery #8B). Pipe an upright dollop of buttercream on top of each larger puff. Place a small, filled puff on top of buttercream, pressing down gently to secure to bottom puff. Serve immediately, or place in an airtight container, refrigerate, and serve within a day.
Just before serving, dust with confectioners’ sugar, if desired.
Recipe by TeaTime Magazine at https://teatimemagazine.com/royal-religieux/