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Position rack in middle of oven. Preheat oven to 350°F/180°. Coat the inside of a 10 inch (25 cm) springform pan with nonstick spray; line the bottom with parchment paper and spray parchment, too.
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Whisk melted butter with sugars in a large mixing bowl. Whisk in egg and yolk until smooth and well combined, then whisk in vanilla. Fold in flour, baking soda and salt until a few floury streaks remain, then fold in 1 cup (170 g) of the mini chocolate chips and 1 tablespoon of the sprinkles. Spread batter into prepared pan, leveling the top with an offset spatula.
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Bake for about 25 to 30 minutes or until Happy Birthday Cookie is golden brown and a toothpick inserted in the center just tests clean. Cool pan completely on rack. Unmold cookie from pan and use a broad sturdy spatula to pry it from the parchment paper and place on a serving platter.
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Melt remaining 1/4 cup (43 g) of chocolate chips in microwave or over simmering water. Spread melted chocolate around edge of cooled cookie and immediately sprinkle the rainbow jimmies on top so that they adhere, gently pressing into place.
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TA DA! Light the candles and sing Happy Birthday!