1-First Step: Begin by creaming the softened butter with both sugars. In a large mixing bowl, beat together 1 cup softened butter, 1 cup granulated sugar, and 1/2 cup brown sugar using an electric mixer on medium speed. Continue beating for about 2 minutes until the mixture is light and fluffy. This process incorporates air into the mixture, which helps create a lighter texture in your cookie bars. Scrape down the sides of the bowl as needed to ensure all ingredients are well combined.

2-Second Step: Add the eggs and vanilla to the creamed butter mixture. Crack in 2 large eggs one at a time, beating well after each addition. Then add 2 teaspoons of vanilla extract and mix until fully incorporated. The eggs provide structure and richness to the bars, while the vanilla adds that classic flavor that makes these taste reminiscent of birthday cake. For best results, use room temperature eggs to prevent them from curdling when added to the butter mixture.

3-Third Step: In a separate medium bowl, whisk together the dry ingredients. Combine 2 1/2 cups all-purpose flour, 1 teaspoon baking powder, and 1/2 teaspoon salt, mixing well to ensure the leavening is evenly distributed. This step prevents pockets of baking powder or salt in the finished bars, which could affect both flavor and texture.

4-Fourth Step: Gradually add the dry ingredients to the wet mixture. With the mixer on low speed, slowly add the flour mixture to the butter and egg mixture, mixing until just combined. Be careful not to overmix at this stage, as overmixing can develop the gluten in the flour, resulting in tougher cookie bars. The dough should be thick and slightly sticky but still manageable.

5-Fifth Step: Fold in the rainbow sprinkles. Gently fold 1/2 cup of rainbow sprinkles (jimmies) into the dough using a spatula or wooden spoon. Using an electric mixer for this step might break the sprinkles or cause them to bleed color into the dough, creating a muddy appearance. Gentle folding preserves their shape and ensures even distribution throughout the bars.

6-Sixth Step: Spread the dough evenly in your prepared 9×13-inch pan. The dough will be thick, so you may need to use your hands or the back of a spatula to press it into an even layer. Try to keep the thickness consistent across the entire pan to ensure uniform baking. If the dough sticks to your hands, lightly dampen them with water or coat them with a little flour.

7-Seventh Step: Bake the bars in your preheated 350°F (175°C) oven for 25-30 minutes. The bars are done when the edges are lightly golden and the center appears set but still slightly soft. A toothpick inserted in the center should come out mostly clean with a few moist crumbs. Avoid overbaking, as this will result in dry bars rather than the desired chewy texture.

8-Eighth Step: Remove the pan from the oven and place it on a wire rack to cool completely. This is a crucial step that cannot be rushed. The bars need to cool completely before frosting, otherwise the frosting will melt and slide off. Allow at least 1-2 hours for cooling, or place the pan in the refrigerator for faster results. The bars will also continue to set slightly as they cool.

9-Ninth Step: While the bars are cooling, prepare the frosting. In a medium bowl, beat 1/2 cup softened butter until smooth and creamy. This usually takes about 2 minutes with an electric mixer. Starting with softened butter ensures a smooth, lump-free frosting.

10-Tenth Step: Gradually add 3 cups of powdered sugar to the butter, mixing on low speed initially to prevent a cloud of sugar. Once the sugar is incorporated, increase the speed to medium and beat for 2 minutes until the mixture is smooth and creamy.

11-Eleventh Step: Add 1/4 cup heavy cream and 1 teaspoon vanilla extract to the butter and sugar mixture. Beat on medium-high speed for another 2-3 minutes until the frosting is light, fluffy, and spreadable. The heavy cream creates a rich, milkshake-like consistency that complements the cookie base perfectly. If the frosting seems too thick, add a little more cream, 1 teaspoon at a time. If it’s too thin, add more powdered sugar until you reach the desired consistency.

12-Final Step: Once the cookie bars are completely cool, spread or pipe the frosting evenly over the top. For a milkshake look, transfer the frosting to a piping bag fitted with a large star tip and pipe swirls across the surface of the bars. This technique creates that soft-serve ice cream appearance that makes these bars so special. Immediately sprinkle additional rainbow sprinkles over the frosting while it’s still fresh so they adhere well. For clean cutting, place the frosted bars in the refrigerator for 30 minutes to firm up. Then, using the parchment paper overhang, lift the bars from the pan and place them on a cutting board. Use a sharp knife to cut them into 24 equal-sized bars, wiping the knife clean between cuts for neat edges. These Funfetti Milkshake Cookie Bars are now ready to serve and enjoy!