1-First Step: Preheat and Prep Pan Preheat your oven to 350°F. Grease a 9×13-inch baking pan with butter or non-stick spray, then line with parchment paper for easy removal. This prevents sticking and ensures clean slices for your funfetti cookies. For gluten-free adaptations, use a silicone liner to avoid paper tears.

2-Second Step: Cream the Butter and Sugars Use a stand mixer or hand mixer. Beat 1 cup softened unsalted butter, 1 cup brown sugar, and ½ cup granulated sugar on medium speed for 2 full minutes until light and fluffy. Scrape down the bowl sides midway. This step aerates the dough, creating those signature chewy milkshake cookies. If using low-calorie options, blend in sugar substitutes here for even mixing.

3-Third Step: Add Eggs and Vanilla Add 2 large eggs one at a time, beating well after each. Mix in 2 tsp vanilla extract until smooth, about 1 minute. The mixture may look curdled briefly, but it will come together. For vegan versions, prepare flax eggs ahead and let them gel for 5 minutes before adding. This binds everything perfectly.

4-Fourth Step: Prepare Dry Ingredients In a separate bowl, whisk 2¼ cups all-purpose flour, 1 tsp baking soda, ½ tsp salt, and the optional small box instant vanilla pudding mix. Spoon flour into the measuring cup to avoid packing, which prevents dense bars. Gluten-free bakers, whisk in xanthan gum now. This dry mix ensures even distribution for tender cookie bars.

5-Fifth Step: Combine Wet and Dry Gradually add dry ingredients to the wet mixture on low speed until just combined, about 1 minute. Do not overmix to keep the texture soft. The dough will be thick and scoopable. For dietary tweaks, this is where oat flour shines for extra nutrition without changing flavor much.

6-Sixth Step: Fold in Sprinkles Gently fold in ½ cup rainbow sprinkles with a spatula. Use jimmies, not sanding sugar, to avoid bleeding color. This step adds the fun to your funfetti bars and sprinkle cookies. Taste a tiny bit of dough to check seasoning, adjusting salt if needed.

7-Seventh Step: Spread in Pan Transfer dough to the prepared pan. Use wet hands or an offset spatula to spread evenly to edges. Aim for ½-inch thickness in the center. Press gently to compact sprinkles. For even baking, tap the pan on the counter twice.

8-Eighth Step: Bake to Perfection Bake at 350°F for 25-30 minutes. Edges turn golden, center sets but stays soft. Rotate pan halfway for uniform browning. Gluten-free may need 2-3 extra minutes. Cool in pan on a wire rack for 30 minutes, then fully at room temp for 1 hour. This prevents crumbling.

9-Ninth Step: Make the Frosting Beat 8 oz cream cheese until smooth, 1 minute. Gradually add 2 cups powdered sugar and ¼ cup milk, mixing until creamy. For dairy-free, use vegan alternatives. Spread over cooled bars. Dairy-free milk works best here without curdling.

10-Final Step: Garnish, Chill, and Serve Sprinkle extra rainbow sprinkles on top. Chill in fridge for 30 minutes to set frosting. Lift out using parchment, slice into 20 squares. Serve at room temp for best chew. Pairs great with milk, mimicking a true milkshake experience in birthday cake cookie bars.