Whisk the sour cream into the buttermilk in a small bowl.
In a large bowl, stir together the flour, baking soda, baking powder, and salt, until completely mixed. Cut in the shortening with a fork until coarsely mixed in. (Should look crumbly.)
Pour the buttermilk mixture into the flour mixture and fold together with a spatula until just combined, and then hand-knead until smooth.
On a clean floured surface, use a rolling pin to roll dough out to about 1 inch thickness. Dip the rim of a drinking glass into flour and cut the biscuits. Reroll excess and cut until all dough is used.
Brush off excess flour from the biscuits and place them onto an ungreased baking sheet. Bake biscuits on center rack for 15 minutes, or until golden brown and risen. Brush with melted butter if desired.