In a medium nonstick skillet, combine 1/2 cup water, 1/2 cup sugar and the lemon juice over medium heat. Cook, stirring, until the sugar dissolves. Add the rhubarb and simmer until very thick, stirring to break up the rhubarb and scrape the skillet sides, about 10 minutes. Transfer to a bowl and let cool.
Preheat the oven to 400 degrees . Grease a baking sheet. In a bowl, whisk together the flour, 3 tablespoons sugar, the baking powder and salt.
Using an electric mixer, beat the cream on medium-high speed to form soft peaks. Beat in the vanilla. Make a well in the flour mixture, add the whipped cream and stir with a fork until a dough begins to form. With floured hands, knead the dough in the bowl until almost all the flour is incorporated. Place the dough on a lightly floured surface and pat into a 1/2-inch-thick round.
Using a 2 1/2-inch cookie cutter, cut 8 rounds, patting scraps together. Brush the biscuits with extra cream and sprinkle with sugar; place on the baking sheet. Bake until golden-brown, about 15 minutes. Serve warm with the butter and rhubarb jam.