Heat oven to 375 degrees F. Butter and flour shell-shaped madeleine molds. Tap out excess. In a medium bowl, sift together flour, baking powder, and cinnamon; set aside. In the bowl of an electric mixer fitted with the whisk attachment, beat eggs and sugar until creamy. Add flour mixture and milk; mix until smooth. Add butter and beat until creamy. Cover with plastic; chill for 20 minutes.
Transfer mixture to a medium pastry bag fitted with a coupler. Pipe mixture into prepared molds. (Alternatively you can use 2-inch paper baking cases.) Transfer to oven and bake until golden brown, about 9 minutes. Remove from molds immediately and cool on a wire rack.
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