Making the Raspberry-Rosemary Compote:
In a small saucepan, combine your raspberries, sugar, and fresh rosemary. Heat over medium flame, gently breaking the raspberries apart with a spoon as they warm up.
After about 3 minutes, reduce the heat to low. Let the mixture simmer, bubble, and thicken for 10-15 minutes, stirring occasionally. It should become slightly gel-like.
Carefully spoon the raspberry mixture into a fine-mesh strainer placed over a bowl. Press the compote through the strainer with the back of a spoon to remove seeds and rosemary leaves, leaving you with a smooth, flavorful compote.
If your compote is too runny, return it to the saucepan and simmer for another 5 minutes, or stir in 1-2 teaspoons of cornstarch dissolved in a tiny bit of water to thicken it. Cover the bowl with plastic wrap and chill in the fridge until ready.
Preparing the White Chocolate Cheesecake Mousse:
Set up a double boiler (or use a heat-proof glass measuring cup placed in a saucepan with a little water). Add your chopped white chocolate and melt it over medium heat, stirring occasionally, until completely smooth. Be careful not to let any water get into the chocolate!
In a large mixing bowl, whip the heavy cream with an electric mixer on high speed until it forms thick, soft peaks.
Gently fold in the whipped cream cheese, vanilla extract, and sugar into the whipped cream until well combined.
Pour the slightly cooled melted white chocolate into your mousse mixture. Mix thoroughly until everything is smooth and uniform.
Transfer the mousse to a container with a lid and refrigerate for at least 4 hours, or until firm.
Assembling Your Valentine's Day Treat:
Fit a pastry bag with a tip and seal the end (just above the tip) with a twist tie. Fold the top of the bag down and place it in a tall cup to hold it open and steady.
Spoon a generous tablespoon of the chilled white chocolate mousse into the bottom of the pastry bag, pushing it down. Then, add about a teaspoon of the raspberry-rosemary compote over the mousse. Don't worry about being too neat – a little messiness creates a beautiful marbled effect!
Continue layering mousse and compote until your pastry bag is full.
Remove the twist tie from the bottom of the bag and pipe the beautiful layered mousse into your serving glasses. Serve immediately for best results, or refrigerate until you're ready to share this lovely dessert!