How to Make It
Purée the raspberries in a food processor, then push them through a fine-mesh sieve to remove the seeds. You should have 1 cup.
Put the berry purée in a small saucepan and stir in the sugar. Place over medium heat, bring to a simmer, and cook, stirring occasionally with a rubber spatula, until the purée reduces by half (1/2 cup), about 15 minutes.
Put the cream in a small saucepan. Place over medium heat, bring to a boil, and remove from the heat.
Add the raspberry purée and the chocolate to the hot cream. Whisk until the chocolate is melted.
Add the butter and whisk vigorously until smooth. Add the white chocolate liqueur and whisk to incorporate.
Use immediately or let cool to room temperature before using. Or, pour the sauce into a jar, cover, and refrigerate.
Cook the pasta as per the recipe's or box's instructions. Drain and toss with enough sauce to coat the pasta (add more or less depending on how saucy you want it). Serve, topped with extra raspberries.