In a stand mixer fitted with a paddle attachment, beat the butter and brown sugar together until creamy, similar in consistency to peanut butter. This should take about 3–5 minutes. Scrape the sides of the bowl as needed.
Add in vanilla extract, salt, and cornstarch. Beat just until combined. Add flour and beat just until incorporated—avoid overmixing. The dough may seem a bit crumbly at first, but it will come together with some mixing/kneading.
Form the dough into one large ball, then divide it in half. Shape each half into a log about 9 inches long and roughly 2 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 liner. Unwrap one dough log at a time from the fridge and slice each dough log into 1/4-inch thick cookies (thin!). Use the tip of a piping tip to cut out the center. Place the cookies on your prepared sheet pan, spacing them 1 inch apart.
Bake until the edges are just starting to lightly brown, about 8–12 minutes. Keep a close eye on them to prevent overbaking.
Remove the cookies from the oven and transfer them to a cooling rack. Let them cool completely. Repeat with the rest of the dough.
In a microwave-safe bowl, combine 1 cup chocolate chips with 2 tsp vegetable oil. Microwave in 20-second increments, stirring for 15 seconds between each burst, until chocolate is melted and smooth. Dip the bottom half of each cooled cookie into the melted chocolate, tapping the cookie to the edge of the bowl to even out the chocolate and let the excess drip down. Place the dipped cookie chocolate side up on a lined sheet pan. Let chocolate fully set/harden (pop in the fridge or freezer to speed this up!). Dip all the bases of the cookies.(Melt remaining 1 cup chocolate chips and remaining 2 teaspoons vegetable oil as needed. Rewarm chocolate as needed in short microwave bursts.)
Line a tall cup with a resealable plastic bag. With whatever chocolate is left over, use a spatula to transfer it into the plastic bag, seal the bag without air, then snip off the tip and drizzle it over the other side of the cookies. Allow the chocolate to set at room temperature (so they aren’t cold!) before serving.
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Notes
Note 1: Over-measuring flour can lead to dry, crumbly cookies, while under-measuring will cause the cookies to spread and lose shape. Be sure to level off the top of the measuring cup with the flat side of a table knife for the most accurate measurement. Or best case—use a food scale to measure the flour (we’re looking for 325 grams).Note 2: Go for the highest quality chocolate chips you can find. The quality affects not just the melt but also the flavor. My family loves milk chocolate, but semi-sweet or dark chocolate are excellent alternatives. You could even try white chocolate for a unique twist.Storage: Let cookies cool completely, then store in an aitight container at room temperature for about 2–3 days.