Prepare the Dough: Using a large mixing bowl, beat together the softened butter, granulated sugar, and packed light brown sugar until the mixture is creamy and fluffy, about 2-3 minutes on medium-high speed.
Add Wet Ingredients: Mix in the room temperature eggs and vanilla extract until well combined and the mixture is smooth. Scrape down the sides of the bowl as needed to ensure everything mixes evenly.
Combine Dry Ingredients: In another bowl, combine the all-purpose flour, baking powder, baking soda, and salt, whisking until thoroughly mixed.
Mix Wet and Dry Ingredients: Gradually incorporate the dry ingredients into the wet ingredients in the mixing bowl, mixing on low speed until just blended. To keep the cookies soft, avoid mixing too much.
Fold in Sprinkles and Chocolate Chips: Gently fold the rainbow sprinkles and white chocolate chips into the cookie dough using a spatula, ensuring they are evenly distributed without overworking the dough.
Chill the Dough: Wrap the bowl with plastic wrap or place the dough in an airtight container. Place in the refrigerator for at least 1 hour, or up to 24 hours. Chilling helps the dough firm up for easier handling and prevents excessive spreading during baking.
Preheat the Oven: About 15 minutes before baking, preheat your oven to 350°F (175°C). Place parchment paper or silicone baking mats on baking sheets.
Shape the Cookies: Scoop about 1.5 tablespoons of dough per cookie using a cookie scoop or spoon. Roll each portion of dough into a smooth ball using your hands.
Coat in Sprinkles: Roll each dough ball in additional rainbow sprinkles to coat evenly, enhancing both appearance and texture.
Place on Baking Sheets: Arrange the coated dough balls on the prepared baking sheets, spacing them about 2 inches apart. Flatten each ball slightly with the palm of your hand to ensure even baking.
Bake the Cookies: Bake in the preheated oven for 10-12 minutes, or until the edges are set and just beginning to turn golden brown. The centers should still be slightly soft to the touch.
Cooling: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Cooling on the rack helps maintain their texture.
Enjoy: Once fully cooled, serve and enjoy these delightful Birthday Cake Cookies with their soft, chewy texture and colorful appearance.