In food processor, finely grind freeze-dried raspberries (you should have scant 1/2 cup).  In large bowl, whisk together flour, baking powder, raspberry powder, and salt. In another large bowl, using electric mixer, beat butter and sugar until light and fluffy, about 3 minutes. Beat in egg and then vanilla. Reduce mixer speed to low and gradually add flour mixture, mixing just until incorporated. Shape dough into two 2-inch-square logs. Wrap and freeze for 20 minutes. Heat oven to 350°F. Line baking sheets with parchment paper. Slice into 1/8-inch-thick squares. Place on prepared sheets.  Bake, rotating positions of baking sheets halfway through, until cookies are light golden brown around edges, 10 to 12 minutes. Let cool on sheets 5 min. before transferring to wire racks to cool completely. Drizzle with melted white chocolate and sprinkle with crushed freeze-dried raspberries.