In large bowl, whisk flour, cornstarch, baking powder, baking soda, and salt.
With mixer on medium-high speed, beat butter and sugars until creamy and fluffy, about 5 minutes. Add eggs one at a time, beating well in between additions. Beat in vanilla. Add flour mixture. Beat on low until just combined. Add chips and beat until evenly distributed. Transfer dough to gallon-size resealable plastic bag; refrigerate until firm.
Preheat oven to 350 degrees F. Break off golf-ball-size piece of dough; roll into ball (should be about 1 1/2 inches wide) and place in one cup of nonstick popover pan. Repeat for remaining 5 cups. Using rounded end of wooden spoon and fingertips, press dough in the center, then up side of cup to form very thin, even layer of dough. Patch up any tears with bits of extra dough. Repeat with remaining dough balls.
Bake 8 minutes. Using same wooden spoon end, gently press dough down and against sides of pan to deflate. Bake another 2 minutes. Repeat pressing to deflate. Bake another 2 minutes or until edges are golden brown and crisp. Transfer to wire rack. Press to deflate once more.
Using small icing knife, gently loosen sides of cookies from pan. Cool 10 minutes. Loosen sides of cookies again. Place wire rack on top of pan. Holding rack against pan, invert cups onto rack. Cool completely. Repeat process once more with remaining dough and cooled popover pan (you will have dough left over, which you can refrigerate up to 3 days or freeze up to 1 month for another use).
With small pastry brush, paint insides of cookies with thick layer of melted chocolate. Let stand until chocolate is set, about 1 hour. Serve filled with milk, whipped cream, pudding, etc. Once filled, serve cookies within 5 minutes. Store unfilled cookies in airtight container at room temperature up to 3 days.
This recipe first appeared on GoodHousekeeping.com.