Funfetti sugar cookie birthday cake

• 今天是小J6岁生日，做了不一样的蛋糕希望他喜欢（这蛋糕小J多数都有帮忙哦）。这蛋糕有点像饼干又有点像蛋糕，放一球雪糕有好吃咧！糖的份量减少许多，还是会甜，下次要减白巧克力了。
• 食谱取自Sally's baking addiction
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• Ingredients:
• 1/2 cup (115g) unsalted butter, softened to room temperature
• 3/4 cup (150g) granulated sugar  （我用100g）
• 1 large egg + 1 egg yolk
• 2 teaspoons vanilla extract   （没放）
• 1 and 1/2 cups (190g)  all-purpose flour
• 1/4 teaspoon salt
• 1 teaspoon baking powder
• 1/2 teaspoon baking soda
• 1 and 1/2 teaspoons cornstarch
• 2/3 cup (120g) white chocolate chips    （只放80g）
• 1/2 cup (80g) rainbow sprinkles   （只放30g）
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Directions:

1. Preheat oven to 350°F (177°C). Spray a 9-inch pie dish or cake pan with nonstick spray. Set aside.
2. Make the cake: In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Get it nice and smooth, then add the sugar on medium speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down the sides as needed.
3. In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts. The dough is quite thick. Once combined, gently fold in the white chocolate chips and 1/2 cup sprinkles using a rubber spatula (or by mixing on low speed).
4. Press the cookie dough evenly into the prepared pan. Bake for 25-30 minutes or until very lightly browned on top. You want the cookie cake to be extra soft in the center, so careful not to overbake. Allow the cake to cool completely on a wire rack at room temperature before frosting. The center will slightly sink - that's normal.