To prepare the rhubarb-blackberry compote: In a small saucepan combine the rhubarb, blackberries, water, sugar and vanilla and bring to a boil over high heat. Lower the heat and simmer very gently until the rhubarb is tender, 10 minutes. Let cool to room temperature.
To prepare the shortcakes: Preheat oven to 350. Spread 1 cup of the almonds on a baking sheet or in a shallow pan. Bake, stirring once or twice, until lightly browned and fragrant, 5 to 10 minutes. Let cool.
Increase oven temperature to 400.
In a large bowl, combine flour, sugar, baking powder, baking soda and salt; stir. With a pastry blender, two knives or your fingers, cut in the butter until the mixture resembles coarse breadcrumbs. Stir in the toasted almonds. Make a well in the center of the mixture; add the buttermilk, vanilla and almond extracts. Stir to combine just until the dough comes together, adding more buttermilk, 1 tablespoon at a time, if the mixture seems too dry. (The dough should be a little crumbly.)
Turn dough out onto a lightly floured surface and knead very lightly to pull together. Pat into an 8-inch square about ½- inch thick. Do not overwork the dough.
Cut dough into 12, 2x2 ½-inch rectangles and arrange ½ inch apart on a lightly floured or parchment lined baking sheet. Brush lightly with cream and sprinkle with additional sugar and the remaining ¼ cup raw almonds.
Bake until lightly golden, 10 to 14 minutes. Cool 5 minutes on a wire rack before serving.
To assemble each serving, halve warm shortcake horizontally. Place bottom half on a serving plate and spoon over some of the rhubarb-blackberry compote. Top with a generous dollop of whipped cream and the remaining shortcake half.
Yields 12 shortcakes; serves 6 to 12