1. Preheat the Oven and Prepare Baking Sheets

• Preheat your oven to 350°F (175°C).
• Line 2 large baking sheets with parchment paper or silicone baking mats. Avoid greasing the pans—parchment helps prevent excess spread and burning.
• Prepare space in your freezer or refrigerator for chilling dough balls.

2. Mix the Dry Ingredients

• In a medium bowl, whisk together the flour, baking soda, salt, and cream of tartar.
• Set aside to incorporate later.

3. Cream the Butter and Sugar

• In a large bowl using a stand mixer or handheld mixer, beat the softened butter and sugar on medium-high speed for 3 to 5 minutes.
• The mixture should become light and fluffy. This step incorporates air, which contributes to the chewy texture and volume.
• Scrape down the sides of the bowl as needed.

4. Add the Egg and Vanilla

• Beat in the egg and vanilla extract until fully combined.
• The mixture may look slightly curdled at first—keep mixing until smooth and consistent.

5. Combine Wet and Dry Ingredients

• Reduce mixer speed to low. Gradually add in the dry ingredients.
• Mix just until no flour streaks remain; avoid overmixing, which can lead to tough cookies.

6. Fold in the Sprinkles

• Gently stir in the rainbow sprinkles with a spatula or wooden spoon.
• Be careful not to overmix—the color from some sprinkle types may bleed if overworked.

7. Form the Cookie Dough Balls

• Scoop out ¼ cup-sized portions of dough and roll into balls (around 65–70g each).
• Place on a prepared baking sheet, leaving at least 4 inches of space between them—these cookies spread significantly.

8. Chill the Dough Balls

• Chill the formed dough balls in the freezer for 15 minutes or refrigerator for 30 minutes.
• This step prevents the cookies from spreading too quickly and losing shape.

9. Bake and Bang

• Bake 1 sheet at a time for about 10 minutes.
• Open the oven and carefully lift the pan about 3 inches and let it drop onto the oven rack or a heat-safe surface. This helps create the ripples.
• Return the pan to the oven and repeat the pan-drop every 2 minutes until the cookies are golden around the edges—typically a total bake time of 14 to 16 minutes.
• The centers will appear slightly underdone—that’s perfect.

10. Cool the Cookies

• Let cookies cool on the baking sheet for 5 minutes to firm up.
• Transfer to a wire rack to cool completely.
• The ripples will set as they cool, giving a crisp edge and soft center.