Prepare the Raspberry Powder:
Place the freeze-dried raspberries in a food processor and pulse until they turn into a fine powder. Sift the raspberry powder through a sieve into a small bowl to remove the seeds. Set aside.
Make the Truffle Base:
In a large heatproof bowl, combine the white chocolate chips and butter.
In a small saucepan, heat the heavy cream over low heat until it begins to simmer (do not let it boil).
Pour the hot cream over the white chocolate and butter. Let it sit for 5 minutes. Slowly stir the mixture until the chocolate and butter are melted and fully combined.
Incorporate the Raspberry Powder:
Add ⅓ cup of the raspberry powder to the white chocolate mixture and stir until well mixed. Taste the mixture and adjust the raspberry flavor by adding more powder if desired.
Cover the bowl with plastic wrap and refrigerate for about 2 hours, or until the mixture is firm enough to shape into balls.
Shape the Truffles:
Line a baking sheet with parchment paper. Once the mixture is firm, use a heaping teaspoon to scoop out portions and roll them into balls with your hands. Place the truffles on the prepared baking sheet. Continue shaping until you have approximately 24 truffles.
Refrigerate the truffles for 30 minutes.
Coat the Truffles:
Melt the bittersweet chocolate in a large bowl by microwaving in 30-second intervals at 50% power, stirring between each interval. Alternatively, melt it over a double boiler, stirring frequently.
Dip each truffle into the melted chocolate using a dipping tool or two forks. Tap off any excess chocolate, then return the truffles to the tray to set.
Decorate (Optional):
Before the chocolate coating sets, sprinkle some of the remaining raspberry powder or chocolate jimmies on top of the truffles for decoration.
Allow the chocolate to fully set before serving.