Frosted Birthday Cake Cookies

In a medium mixing bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or in a large bowl with an electric hand mixer, cream the unsalted butter and granulated sugar for a few minutes until light and creamy. Add in the eggs and vanilla extract and mix until well combined.
Add in the flour mixture and mix on low speed until the cookie dough comes together. Fold in the rainbow sprinkles until they are well dispersed throughout the cookie dough.
Portion the cookie dough using a large cookie scoop (3 tablespoons) and place on a parchment-lined baking sheet pan. Chill the cookie dough balls for 30 minutes.
While the cookie dough is chilling, preheat the oven to 350℉ (177℃). Line baking sheet pans with silicone baking mats or parchment paper.
Place the chilled cookie dough balls on the prepared baking sheet pans, allowing room for spreading. Bake the cookies for about 12-13 minutes until the edges are set and the centers are slightly underbaked. Cool the cookies on the pan for 2 minutes and then transfer to a wire rack to cool completely.

First, heat-treat the white cake mix. Pour the cake mix into a microwave-safe bowl. Heat the cake mix in the microwave for 30 seconds and then stir. Repeat this process until the cake mix reaches 160℉ (71℃), about 1 ½ minutes total. Run the cake mix through a fine mesh strainer and then set aside to cool.
In the bowl of a stand mixer fitted with a paddle attachment or in a large mixing bowl with an electric hand mixer, whip the room temperature unsalted butter for 5 minutes until light, pale, and creamy.
Add in half of the powdered sugar and mix on the lowest speed until the sugar is combined with the butter. Add the remaining half of the powdered sugar and the heat-treated white cake mix and continue to mix on low speed until fully combined.
Add in the heavy cream, vanilla extract, and salt, and continue to mix on low speed until combined and smooth. Scrape the sides and bottom of the bowl to ensure the ingredients are well incorporated.
Increase the speed on the mixer to medium and whip the buttercream for about 2 to 3 minutes until it is smooth and creamy. Turn the mixer to low speed and mix the buttercream for several minutes to remove any air bubbles.
Optional: To smooth out the buttercream, melt ½ cup of the frosting in the microwave for 15 seconds. Pour the melted frosting back into the mixing bowl and mix the frosting on low speed until the buttercream is super smooth and creamy.

Preheat the oven to 350℉ (177℃). Line a baking sheet pan with parchment paper.
In a medium mixing bowl, whisk together the cake flour, granulated sugar, light brown sugar, baking powder, and salt until well combined. Add in the vegetable oil and vanilla extract and stir until the dry and wet ingredients are fully incorporated and large clusters form. Add in the rainbow nonpareil sprinkles and stir until they are well dispersed throughout the mixture. Break up any larger clusters with a spoon.
Spread the birthday cake crumb mixture onto the prepared baking sheet pan. Bake the crumbs for 10 minutes. After baking, cool the birthday cake crumbs to room temperature and then break them up into small crumbs.

Fill a piping bag fitted with a Wilton 1A piping tip with the cake batter buttercream frosting. Pipe a swirl of the frosting onto the cookies and spread evenly with an offset spatula. Top the cookies with a generous amount of the birthday cake crumbs.

Storing & Serving:

Room temperature: Store the frosted cookies in an airtight container at room temperature for 2 to 3 days.
Refrigerator (cookie dough): Chill the cookie dough balls in the refrigerator for 30 minutes. Transfer the cookie dough balls to a resealable storage bag and store in the refrigerator for up to 2 weeks. Bake as directed.
Refrigerator (frosted cookies): Store the frosted cookies in an airtight container in the refrigerator for 3 to 4 days. Serve at room temperature for the best taste and texture.
Freezer (cookie dough): Store the portioned cookie dough in a resealable storage bag in the freezer for up to 3 months. To use, thaw the cookie dough for an hour and bake as directed.
Freezer (frosted cookies): Store the baked and frosted cookies between sheets of parchment paper in a storage bag or airtight container in the freezer for up to 3 months.

Recipe Tips: Scroll up the blog post to see my expert cookie baking tips and frequently asked questions to make the most out of this recipe.

Note: Metric conversions and nutritional information are calculated automatically. I cannot guarantee the accuracy of this information.