Place the freeze-dried raspberries into a food processor and blend until a fine powder forms.

Set a fine-mesh sieve over a small bowl. Sift the raspberry powder to remove seeds or larger bits. Set the powder aside, reserving about 2 teaspoons for garnish.

Place the chopped white chocolate and butter into a large heatproof bowl.

In a small saucepan, heat the heavy cream over low heat until it just begins to simmer; do not let it boil.

Pour the hot cream over the white chocolate and butter. Let it sit for 5 minutes without stirring.

Gently stir the mixture until smooth and fully combined and melted.

Add the raspberry powder, one tablespoon at a time, whisking well after each addition.

Cover the bowl tightly with plastic wrap and refrigerate for 2 hours, or until the mixture is firm enough to scoop.

Line a baking sheet with parchment paper.

Using a 1-tablespoon scoop, portion out the mixture into balls, placing each on the lined baking sheet.

Chill the balls in the refrigerator for 30-45 minutes, or until firm.

When ready, melt the chopped bittersweet chocolate using a double boiler or in the microwave in 20-second bursts, stirring between each until smooth.

Remove the truffles from the refrigerator in small batches. Working one at a time, gently drop each truffle into the melted chocolate, making sure it is completely covered.

Using a fork, lift out the chocolate-covered truffles and slide them onto the parchment-lined baking sheet using a toothpick or another fork.

While the coating is still wet, sprinkle a little reserved raspberry powder over the top for garnish, if desired.

Let the truffles set at room temperature for about 1 hour, or refrigerate for 15 minutes to speed up setting.

Store in an airtight container in the refrigerator for up to 1 week, or freeze for longer storage.