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Preheat oven to 350oF (180oC)
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In a bowl, mix the butter and sugar together until creamed.
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Mix in the egg and vanilla.
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Stir in flour, baking soda, salt, cream of tartar, and sprinkles until just combined. Do NOT overmix. You will notice this is a soft dough
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Place the dough in the center of the prepared cookie sheet into a tall ball (3 inches wide).
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Bake for 14-16 minutes until the edges are browned. The center may appear undone but that's what you want. It will firm up. Allow to cool completely on the baking sheet. Enjoy!
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*To get 1 Tbsp egg, crack egg into a small bowl, whisk until yolk and white are well blended, then measure out 1 Tbsp.