In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt. Add the cold cubed butter and use a pastry cutter or fork to cut it into the dry ingredients.
In a separate mixing bowl, whisk together the heavy cream, egg, and vanilla extract. Add the wet ingredients and stir until just combined, then gently fold in your favorite mix-ins.
Scoop the dough onto a lightly floured surface and work it into a ball, then flatten into a 7-inch circle. Cut the dough into 8 equal-sized pieces and place them on a lined baking sheet.
Make the glaze by whisking together the powdered sugar, milk, and vanilla extract. Once the scones have cooled, top them with the mixture and allow the glaze to harden.