Preheat oven to 325 degrees. Cut out twenty-four 5 1/2-by-5 1/2-inch squares of parchment paper. Fit each square of parchment into a standard muffin cup so the edges stick out from the muffin cup; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat butter until softened. With the mixer on medium, add eggs and sugar; beat until well combined. With mixer on low speed, add flour in three parts, alternating with buttermilk and beginning and ending with flour. Stir in vanilla.
Fill prepared muffin cups three-quarters full with batter. Transfer muffin tins to oven and bake until a cake tester inserted into the center of a cupcake comes out clean, 20 to 25 minutes. Transfer to a wire rack and let cupcakes cool completely in muffin cups.
When cupcakes are cool, frost the top of each cupcake with a thin layer of buttercream. Fill a disposable pastry bag fitted with a 3/4-inch St.-Honore decorating tip with remaining buttercream. Pipe three waves on top of each cupcake, each slightly smaller than the next. Sprinkle with turbinado sugar and serve.