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Preheat oven to 425°; combine flour, cocoa powder and baking soda; set aside; in a saucepan over medium-high heat bring to a boil the water, butter,sugar and salt.
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Add the flour mixture and continue cooking and stirring until the dough pulls away from the sides of the pan and a film forms on the bottom.
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Scrape dough into a bowl and beat in eggs and yolk, one at a time, incorporating each thoroughly before adding the next.
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Using a plastic bag with a corner snipped off or a pastry bag with a 3/4" plain round tip, pipe 1 1/2" rounds about 3/4" high onto a parchment lined baking sheet; flatten any pointed tops with a moist finger.
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Bake 15 minutes; reduce heat to 325° and bake until no wet dough remains in center, about 30 minutes; cut into one of the puffs to test, there should be a slight dampness.
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Cool on a wire rack; cut in half horizontally with a serrated knife and fill with choice of filling.