Add the raspberries to a food processor and pulse until they are a fine powder. If you don't have a food processor, place in a sealed bag and crush with a rolling pin.

Add ⅔ of the sugar to a bowl, then sift the raspberry powder into the sugar and discard the seeds. Whisk until well combined.

Place puff pastry on a clean surface, then sprinkle with remaining ⅓ cup of (plain) sugar. Lightly roll out the rectangle with a rolling pin (this also helps the sugar stick to the puff pastry).

Flip over the puff pastry, then sprinkle liberally with raspberry sugar. (NOTE: you may have sugar leftover. Just use enough to fully coat the puff pastry in an even layer.) Press it in with your hands, then use a rolling pin to roll it out. Most store bought puff pastry will be about 10x11 inches, but ours was a bit bigger, so we rolled it to about 13x14 inches.

Fold the longer section of your puff pastry in half to mark the middle point. Then fold each end a little less than halfway to the middle and sprinkle with more raspberry sugar. Make another fold, folding each end towards the middle. Sprinkle on a little more regular sugar. Fold again so the two sides meet and press down gently with a rolling pin. Place in the freezer for 30 minutes.

Preheat oven to 400˚F.

Remove from freezer and cut into ½-inch thick slices. Space about 2 inches apart on parchment paper and bake for 8-10 minutes, then flip with a spatula and cook for another 4-5 minutes, until golden brown and crispy. Let cool completely, then dip in melted white chocolate and sprinkle with crushed freeze dried raspberries.