Preheat oven to 425F (220C). In a large mixing bowl, stir together blueberries, sugars, corn starch and lemon juice until combined. Pour into a 7×11 or 9×9 glass or ceramic baking dish and set aside while you prepare the topping.
Stir ingredients well and then pour into a 7×11 baking dish². Set aside and prepare your blueberry cobbler topping.
For the ToppingIn a separate large mixing bowl, combine flour, sugar, baking powder, and salt and whisk well.
Ue a box grater to shred the butter into small pieces and then add to the flour mixture (alternatively use a pastry cutter to cut the butter into the mixture).
Use a fork to stir in the grated butter until the mixture resembles coarse crumbs.
Add heavy cream and milk and use a wooden spoon or rubber spatula to stir until ingredients are combined (don't over-mix).
Evenly drop and distribute topping over the blueberry filling that you prepared earlier. The biscuit batter does not have to totally cover the blueberries, just do your best to evenly distribute it so that it will cook evenly.
Brush with additional heavy cream and sprinkle lightly with coarse sugar (if you don't have coarse sugar, regular granulated will also work just fine).
Transfer to oven and bake on 425F (220C) for 25-30 minutes or until top is golden brown. Cover cobbler with foil and bake an additional 10-15 minutes or until a toothpick inserted in the center of the biscuit portion comes out clean or with a few dry crumbs, you just need to make sure the biscuit portion is cooked through before serving.
Allow to cool for about 10-15 minutes before scooping and serving warm. Top with vanilla ice cream, if desired.