Position a rack in the center of the oven, and preheat to 375 degrees F. Line 1 large rimmed baking sheet with parchment. Whisk together the flour, baking soda and salt in a small bowl.
Microwave the butter in a large microwave-safe bowl on high until completely melted, about 1 minute. Whisk in both sugars, then the egg and vanilla, until incorporated and smooth; let cool for 5 minutes. Stir in the flour mixture with a wooden spoon until just incorporated. Stir in the chocolate chips.
Scoop the dough onto the center of the prepared baking sheet. Spread the dough with an offset spatula into an 8-inch circle that's 1/2 inch thick. Sprinkle the top of the cookie with the sprinkles (they'll look crowded but will spread out as the cookie bakes). Bake, rotating the baking sheet from back to front halfway through, until golden but still soft in the center, 22 to 25 minutes (the longer the cook time, the crunchier the cookie). Let cool for a few minutes on the baking sheet, then use a large spatula to transfer the cookie (not the parchment) to a rack to cool. Slice the cookie into wedges, and serve.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.) Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.
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