Preheat the oven to 425* F. Grease or parchment line a baking sheet and sprinkle with flour and cornmeal. In a large bowl using a whisk or electric mixer, combine the flour, baking powder, baking soda and salt.
Using a pastry blender or 2 knives, cut the butter/shortening into the dry ingredients. The mixture will resemble coarse crumbs, with no large chunks of butter. If the butter get very soft at this point, refrigerate the mixture for 20 minutes to re-chill. Add the egg to the Smith's buttermilk blend together, then add to the dry mixture stirring just to moisten all the ingredients. The dough will be moist, then stiffen while stirring. It should be slightly shaggy, but not sticky.
Turn the dough out onto a lightly floured surface and knead gently about 6 times, or just until the dough holds together. Roll or pat out the dough into a rectangle no more than ¾ inch thick. Take care not to add too much flour at this point or dough will become tough. Cut with a floured 2 ½ inch biscuit cutter, pushing straight down without twisting. Cut as close together as possible for a minimum of scraps. Pack the scraps together and re-roll the dough to cut out additional biscuits.
Place the biscuits ½ inch apart on the baking sheet. Place the baking sheet on a rack in the center of the oven and bake for 15 – 18 minutes or until golden brown. Let rest a few minutes and serve hot. Can be made ahead and frozen to reheat for later use. If freezing the biscuits, pull from the freezer the night before and warm in a 375 degree oven for 5 – 7 minutes and serve hot.