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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.
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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.
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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.
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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.