How to Make It
Beat butter and ⅔ cup of the sugar with an electric mixer on medium speed until light and fluffy, about 5 minutes. Beat in lemon zest and vanilla. Whisk together flour, cornstarch, and salt in a medium bowl. Gradually add flour mixture to butter mixture, beating at low speed until well combined, about 2 minutes. Shape dough into a disk, and wrap with plastic wrap; chill 1 hour.
Meanwhile, place marmalade and preserves into 2 separate small microwave-safe bowls; microwave both on HIGH until thick and bubbly, about 3 minutes, stirring at 1-minute intervals. Cool completely, about 30 minutes.
Preheat oven to 350°F and line 2 baking sheets with parchment paper. Place remaining ⅓ cup sugar in a shallow dish. Shape dough into 36 balls, about 1 tablespoon each; roll dough balls in sugar, and place 2 inches apart on prepared baking sheets. Press thumb into the center of each cookie, leaving an indentation. Fill half of the cookies with marmalade and the remaining half with plum preserves or jam. Bake until cookies are set but not browned, 12 to 14 minutes. Cool on pans about 3 minutes. Transfer cookies to wire racks to cool completely, about 20 minutes.
Place white chocolate in a small microwave-safe bowl; microwave on HIGH, stirring every 15 seconds, until completely melted and smooth, about 45 seconds. Drizzle cookies with melted chocolate, and let stand until dry and set, about 20 minutes.