Using an electric mixer, cream the granulated sugar and butter on high speed until fluffy, about 5 minutes. Mix in the baking powder and salt at low speed until blended. At medium speed, beat in the egg and vanilla. Add the flour in 2 additions, mixing at low speed, just until incorporated. Pat the dough into a 4 inch diameter disk, wrap in parchment or wax paper and refrigerate until firm, about 30 minutes.
Position racks in the upper and lower thirds of the oven and preheat to 375 degrees . Line a large cookie sheet with parchment. On a lightly floured work surface, roll out the dough 1/4-1/8 inch thick. Cut out 8 cookies using a 3 1/4 inch heart-shaped (or other shape) cookie cutter and transfer to the prepared cookie sheet.
Generously sprinkle the cookies with the sanding sugar. Spritz the cookies with a little bit of water (using a spray bottle or just flick with your fingers) to make the sugar sparkle. Bake until the edges begin to brown, about 10 minutes. Transfer the cookies to a rack to cool completely.
Let the sorbet soften until spreadable. Working on a parchment lined cookie sheet, place a rounded 1/3 cup scoop of softened sorbet onto the unsugared sides of half the cookies. top with the remaining cookies sugared side up and press gently to evenly distribute the sorbet; smooth around the edges with the back of an offset spatula. Freeze until firm, about 30 minutes. Serve, or wrap and freeze for up to 2 weeks.