Prep and line: preheat your oven to 375°F and line two baking sheets with parchment paper.
Cream butter, sugars, and vanilla: cream together the butter, sugar, corn syrup, and vanilla extract for 3 minutes until very light and fluffy on medium-high speed.
Whip the egg: add in the egg and lemon juice, and beat for 1-2 more minutes on medium-high until the batter is smooth, opaque, and slightly fluffy (you can over-whip the batter here and break the mixture, so be sure not to beat longer than 2 minutes).
Mix in dries: add in flour, milk powder, cream of tartar, baking soda, and salt, and mix on low speed until fully combined, then beat briefly on medium-low for just 15-20 seconds. A smooth, slightly sticky dough should form, and be sure to scrape the bowl really well about half-way through mixing.
Scoop and sprinkle: place sprinkles in a fairly small, wide bowl. Scoop dough into 1.5" balls, roll between your hands to fully round if not using an ice cream scoop, then roll in the sprinkle bowl until fully coated (see tips). (An ice cream scoop is really helpful here as this is a bit of a sticky dough to make sure all the sprinkles adhere, but you can use a regular spoon and roll by hand if needed.)
Tray and bake: place cookies on prepared trays, at least 3” apart. Bake for 9-11 minutes, just until they have a hint of golden brown on the edge (you do want these cookies to be a little under-baked for the tastiest texture).
Knock em' down: knock each tray firmly on the counter or stove top immediately after removal from the oven to collapse the dome and get that perfect cookie crackle.
Cool, share, and enjoy: allow cookies to cool as long as you can (so those doughy middles can set up to a perfectly chewy center, and the edges can get deliciously crisp). Then share with anyone and everyone to have a birthday party any day of the week!
Notes
This dough has to be stiff enough to keep its shape and give you that beautiful crackle look, but also sticky enough to hold the sprinkles. If your dough isn't quite sticky enough when you roll it in your sprinkle bowl, you can use your hands to gently press the sprinkles into your dough balls while they’re in the sprinkle bowl, and then just reform them into rounds once fully sprinkled.This recipe was inspired by, and adapted from Momofuku Milk Bar: A Cookbook by Christina Tosi.
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