In a stand mixer fitted with a paddle attachment (or with a hand mixer), beat butter and brown sugar until they’re creamy, similar to a peanut butter consistency, about 3–5 minutes. Scrape the sides of the bowl as needed.
Add vanilla extract, almond extract (if using), salt, and cornstarch. Beat just until combined. Add flour and beat just until incorporated. Avoid overmixing.
Fold in the sprinkles and mix gently. The dough may seem a bit crumbly at first, but it will come together with some mixing/kneading.
Form the dough into 1 large ball, then divide it in half. Shape each half into a log about 11 inches long and roughly 1.5 inches in diameter. Roll it on the counter to help even it out. Wrap each log tightly in plastic wrap and chill in the fridge for at least 2 hours.
Preheat oven to 350°F. Line a sheet pan with parchment paper or a silicone baking mat. Pour the sparkling sugar onto a large flat plate. Unwrap one log of dough at a time, cut it in half, then brush the outside of each half-log with the beaten egg. Roll the dough log in the sparkling sugar.
Slice each dough log into 1/2-inch thick cookies. Place cookies on the prepared sheet pan, spacing 1-inch apart. Bake until the edges are just starting to lightly brown, which should take about 8–12 minutes. Keep a close eye on them to prevent overbaking.
Remove cookies from oven and transfer them to a cooling rack. Let them cool completely. Repeat with the rest of the dough.
Optional White Chocolate Drizzle: In a microwave-safe bowl, combine white chocolate chips and vegetable oil. Microwave in 20-second increments, stirring for 15 seconds between each burst, until chocolate is melted and smooth. Dip half of each cooled cookie into the white chocolate or pour the melted chocolate into a plastic bag, snip off the tip, and drizzle it over the cookies. Allow the chocolate to set at room temperature before serving.
Video
Notes
Note 1: When measuring flour, level off the top of the measuring cup with the flat side of a butter-knife for the most accurate measurement. Or best case—use a kitchen food scale to measure the flour amount (325 grams). Over-measuring flour can lead to dry, crumbly cookies.Note 2: Use sprinkle jimmies, not nonpareils. Here’s a breakdown of different types of sprinkles, but essentially the jimmies won’t dye the cookie dough, and nonpareils are often uncomfortably crunchy.Note 3: This extra-coarse sugar adds a lovely sweetness and crunch to the cookies. If you aren’t able to find it, use sparkling sugar or cane sugar instead.Storage: Keep in an airtight container; use parchment between the chocolate-coated cookies. Freeze dough logs; cut and bake directly from frozen, slightly increasing baking time.