In the large bowl of a stand mixer fitted with the paddle attachment, beat flour, butter, powdered sugar, cocoa powder, cornstarch, salt, and vanilla on medium speed until a thick, smooth dough forms, about 1 minute. Scrape down sides of bowl and continue to beat until there are no pockets of unmixed flour, about 30 seconds more.
Using a rubber spatula, scrape dough onto a sheet of plastic wrap. Pat to a disk, then tightly wrap. Chill at least 2 hours or up to 2 days.
Arrange racks in top and bottom third of oven; preheat to 300°. Line 2 baking sheets with parchment paper. Place another large piece of parchment paper on a work surface. Unwrap dough, place on parchment, then top with another large piece of parchment. Using a rolling pin, roll dough to 1/4" thick. Using a sharp knife or pizza cutter, cut dough into diamond shapes about 3" long and arrange on prepared sheets, spacing about 1 1/2" apart (or you can use a 2" round cookie cutter and cut into rounds). Reroll scraps and cut out more diamond shapes to make 14 to 16 total.
Bake cookies, rotating pans top to bottom halfway through, until firm to the touch, 25 to 30 minutes. Let cool on baking sheet 5 minutes. Using a metal spatula, transfer cookies to a wire rack and let cool completely.
In a small microwave-safe bowl, microwave chocolate chips in 30-second intervals, stirring between each, until melted and smooth. Using a spoon or a piping bag, drizzle chocolate over cooled shortbread. Top with sugar pearls (if using). Refrigerate until set, 15 to 20 minutes.
Make Ahead: Cookies can be made 1 week ahead. Store in an airtight container and refrigerate.