Step 1 Heat the oven to 450 degrees. If using the cranberries, place them in a small bowl and cover with boiling water for 5 minutes to soften. Drain the berries, pat dry and set aside.
Step 2 Meanwhile, into a large bowl sift together the flour, baking powder, baking soda, sugar and salt. Sift these dry ingredients together 4 times; this will lighten the final dough. Cut in the shortening until the mixture resembles coarse meal, then add the buttermilk and the prepared cranberries if using and stir the mixture gently until it forms a sticky dough.
Step 3With floured hands, transfer the dough to a well-floured surface and knead it lightly 8 times. Pat out the dough to one-half inch thickness, and cut out biscuits using a floured 2-inch round cutter. Arrange the biscuits upside down on a greased baking sheet so they just touch each other. (The cutter pinches the edges together, so turning the biscuits over after cutting allows them to rise a bit higher.) Gather the scraps, pat out the dough and cut more rounds in the same manner until the dough is used.
Step 4Bake the biscuits in the middle of the oven until they are golden, 10 to 12 minutes. Serve the biscuits warm.