These shortbread cookies contain ground jasmine tea, which lends a sweet, floral balance to their tart grapefruit-curd filling.
Beat together butter and confectioners' sugar with an electric mixer on medium-high speed until smooth, about 3 minutes. Beat in vanilla. Add flour, 1/2 teaspoon salt, and the tea, and beat on low speed until just combined (do not overmix). Form dough into 2 disks, wrap in plastic wrap, and refrigerate at least 1 hour and up to overnight.
Preheat oven to 325 degrees. Working with 1 disk at a time, roll out dough between pieces of parchment to a 1/8-inch thickness. Freeze until firm, about 10 minutes. Cut out 20 squares with a 2 3/4-inch square cookie cutter, then cut each in half to make 40 rectangles.
Gently press 1 flat side of 20 rectangles in sanding sugar, and arrange on a parchment-lined baking sheet, sugar side up. Bake until pale gold, 16 to 18 minutes. Let cookies cool on a wire rack. Repeat with remaining rectangles.
Spread 1 generous teaspoon curd onto unsugared side of half the cookies, then sandwich with remaining cookies.