In medieval times, posset was a drink made from curdled milk and wine; its modern iteration is a silken custard of cream, sugar and lemon juice. We know there is a scientific explanation for the way the lemon sets the cream and sugar, but we think that a dessert like this, from so few ingredients and in just 15 minutes, is just magic. It does need to set, though; chilled for 3 hours it is just spoonable, overnight, it's firmer and equally spectacular.
And if you have it on hand, put some orange zest into the cream before scalding it, and then strain it out before pouring the cream over the berries.
1/4 cup freshly squeezed lemon juice (about 2 lemons)
Reserve 4 berries for garnish and divide the remaining berries among four 8-ounce mason jars or tempered glasses.
Put 2 cups of the cream and 1/2 cup of the sugar in a small saucepan. Split the vanilla bean, scrape the seeds into the cream and drop in the bean halves. Bring the mixture to a boil over medium to medium-high heat and boil for 4 minutes, stirring occasionally and taking care that it doesn't boil over (which can happen really easily). Remove from the heat, discard the vanilla bean halves and stir in the lemon juice.
Divide the cream among the glasses (the berries will float up to the top). Refrigerate until set, at least 3 hours or overnight.
Before serving, gently whip the remaining 1/2 cup cream with the remaining 1/2 teaspoon sugar, and dollop over the possets. Top each serving with one of the reserved raspberries. (If you serve the possets the next day, you can let them warm for 20 minutes or so at room temperature before serving.)