Preheat oven to 425 degrees.
In a large bowl whisk together flour, baking powder, baking soda, cream of tartar and salt.
Add cold butter and work in with fingers or pastry cutter until crumbly.
In a small bowl whisk together buttermilk, ¾ cup heavy cream and egg together.
Add to flour mixture and and blend until just combined.
Turn out onto a lightly floured surface and roll out until 1 inch thick.
Gently fold dough into thirds and roll out to 1 inch thick.
Using a 4 inch round biscuit or cookie cutter, cut out circles and place 12 onto a baking sheet.
Repeat with remaining dough.
Using ¼ cup of buttercream, brush the tops of the shortcakes and sprinkle with ¼ cup sugar.
Bake for 13-15 minutes or until lightly golden brown on top.
In the meantime, beat remaining whipped cream, sugar and vanilla together on high speed for 5 minutes or until thickened.
Cut cooled shortcakes in half and top with whipped cream and fresh blueberries.