Preheat oven to 425 F. Line a large heavy duty baking sheet with parchment. Sift the flour, 1/3 cup of the sugar, the baking powder, and baking soda into a large bowl. Stir in the salt. Using a pastry blender, a fork, or your fingertips, work the butter into the dry ingredients until the mixture resembles coarse cornmeal.
In a small bowl, beat the egg and heavy cream with a fork. Mix in the buttermilk. Make a well in the centre of the flour mixture and pour in the cream mixture. Mix with fork until the dough is evenly moistened and just comes together; it will still look a little shaggy. Gather the dough and gently knead it three or four times. If the dough seems dry and doesn't form a cohesive mass, work in more cream.
Transfer the dough to a lightly floured surface and roll into a 3/4 - inch disk. Use a 2-1/2 inch biscuit cutter to cut the dough into rounds. Transfer rounds to the prepared baking sheet.
Lightly brush biscuit tops with cream, and sprinkle with sugar. Bake 10-15 minutes @ 425F.