Preheat oven to 350 degrees. Line standard muffin tins with paper liners. In mixer bowl, sift flour, sugar, baking soda, baking powder, and salt. Add eggs, yolk, the warm water, buttermilk, vegetable oil, and vanilla; mix until well combined.
Divide batter among liners, filling 2/3 full. Bake until deep golden brown, about 22 minutes.
Transfer tin to wire rack to cool for about 15 minutes. Remove cupcakes from tin; let cool completely on rack before frosting.
To assemble: Working with a few cupcakes at a time, coat smoothly with about 2 tablespoons of buttercream. Immediately press roses into buttercream to cover the top (use about 20 roses per cupcake).
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