Pre-heat oven to 450°F. Combine all-purpose flour, baking powder, salt and sugar in a large bowl and mix with a fork to combine.
Cut the chilled butter into the dry ingredients using a pastry blender or two butter knives until the butter is pea-sized. Add the mashed potatoes to the mixture and mix with a fork until the potatoes are evenly distributed throughout.
Make a well in the center of the mixture and add beaten egg and buttermilk. Using a wooden spoon or fork, mix until a loose dough forms. Turn out onto floured work surface and knead until the dough comes together.
Pat dough into a square 3/4 of an inch thick and cut into 16 evenly sized portions. Place on lined or greased baking sheet and bake 15-20 minutes until golden brown.