In a non-reactive saucepan combine 3 cups blueberries with 3 tablespoons sugar, lemon juice, and crème de cassis.
Cook over medium heat, stirring occasionally, until the berries pop and release their juices.
In a small bowl, combine the cornstarch and 1 tablespoon cold water. Stir into the simmering blueberry mixture. Simmer 1 minute. Remove from heat and add remaining blueberries. Cool over an ice bath.
Refrigerate until ready to use, up to two days.
Preheat oven to 425 °F. Line a baking sheet with parchment paper.
In a food processor, combine the flour, 1/3 cup sugar, baking powder, baking soda, and salt. Pulse in the chilled butter until the mixture resembles coarse meal. Remove to a mixing bowl.
In a small bowl combine the egg, buttermilk, and ¼ cup cream. Stir into the dry ingredients to form a moist dough. Add additional cream if needed.
On a lightly floured surface gently knead several times. Roll the dough out approximately ¾-inch thick and cut into 8 to 10 rounds, each 2-½ inches in diameter.
Place the shortcakes on the prepared baking sheet. Brush tops with cream and sprinkle with sugar. Bake until lightly golden brown, about 15 minutes.
In a chilled bowl, with a chilled whip, beat 1-½ cups cream until lightly thickened. Add 1-½ tablespoons sugar and beat to medium soft peaks.
Chill until ready to use.
For the assembly:
With a serrated knife, split the warm shortcakes. Set the bottoms on individual serving plates and top with blueberry mixture, divided evenly among the shortcakes, allowing some to run off onto the plate. Top the compote with a generous dollop of whipped cream. Cap with the top half of the shortcake. Sprinkle the plate and the top of the shortcake with powdered sugar and serve.