Begin with a batch of Vanilla-Buttercream Frosting, tinted to your desired shade. Transfer to a pastry bag fitted with a large petal tip. Cut out twenty 2-by-3-inch rectangles of parchment. Adhere 1 parchment rectangle to a flower nail with a dab of buttercream.
1. Holding the piping bag at a 45-degree angle, pipe 1 petal. Pipe a second petal, overlapping the first by 1/2 to 3/4 inch.
2. Making a tighter turn with the nail, overlap the second petal to pipe a third.
3. Sprinkle poppy seeds in the centers of flowers to create the look of stamens. Gently remove parchment from nail, and transfer to a baking sheet. Repeat with remaining parchment and tinted buttercream. Refrigerate poppies for at least 2 hours.
4. Gently remove poppies from parchment using an offset spatula. Place 1 on top of each cupcake.