Preheat oven to 400°.
In a food processor, pulse flour, sugar, baking powder, baking soda and salt together. Add the cubed butter until a coarse meal forms. Pulse in the sour cream and egg yolk until just combined.
Turn the sticky dough out onto a lightly floured surface and knead until the dough comes together into a ball. Gently pat the dough down into a 3/4-inch thick square shape. With a large knife cut the dough square into 4 smaller, equal squares. Then cut each smaller square diagonally, with an X, making 4 smaller triangles.
Transfer the 16 triangles onto a parchment paper lined baking sheet. Brush the tops of each scone with heavy cream and sprinkle it with some coarse sugar. This will create a shiny and crunchy top for the scones. Bake in preheated oven for 12 to 14 minutes until the bottoms of the scones are light brown.
Remove from oven, cool slightly on the sheet pan and then transfer to a wire rack to cool completely.