Sprinkle the sugar on the raspberries and toss to combine. Let sit for 10-15 minutes, until they start to soften and release their juices. Blend the raspberries in a blender, then pour the puree through a fine mesh strainer or cheesecloth to remove the seeds. You should be left with about 3/4 cup of puree. If you have more, save it for another use.
Combine the milk and cream in a medium saucepan and bring to a simmer—you want bubbles to appear along the sides of the pan’s edges, but don’t bring it to a full boil. Remove the pan from the heat and add the chopped white chocolate. Whisk until the chocolate is melted, then add the berry puree, vanilla extract, and salt.
Serve immediately with whipped cream and fresh berries on top. It can be made ahead of time and kept in the refrigerator for up to a week. Reheat it in the microwave or on the stove top.
For the best taste and texture, do not substitute white chips for the chopped white chocolate bar. Chips are usually made with palm oil instead of cocoa butter, and they tend to be hard to melt. If you find chips made with cocoa butter, those are a fine substitute, otherwise, look for white chocolate bars. This makes a thick drink, and if you prefer a thinner one, add more milk until you get a texture you like.