Preheat the oven to 425 degrees. Line baking sheets with Silpats (French nonstick baking mats). In a large bowl combine the flours, sugar, baking powder, and salt. Whisk to combine. Add the butter and using your fingers work the butter into the flour until the mixture resembles coarse meal.
Split the vanilla bean lengthwise and scrape out the seeds. Add the seeds to the heavy cream, and set aside the pod. Add the heavy cream to the flour mixture, and stir to combine.
On a lightly floured surface, pat out dough, 3/4-inch thick. Use a 2 1/2-inch cutter to cut into 12 rounds. Place on prepared baking sheets, leaving about 1-inch between biscuits. Bake until golden brown and cooked through, about 25 minutes.
Transfer to a wire rack to cool. Can be made ahead and frozen, in a resealable plastic bag, for up to 1 month.
In a medium skillet melt the butter over medium heat. Add the sugar and vanilla pod. Cook, stirring, until sugar dissolves, about 2 minutes. Add plums and cook, stirring, until plums are slightly softened and heated through, about 4 minutes. Discard vanilla pod.
Split cooled biscuits horizontally. Top with plums and whipped cream. Serve immediately.