Heat oven to 450 degrees F.
In a large bowl, whisk together the flour, baking powder, baking soda and salt. Cut in shortening with pastry blender until mixture is crumbly.
In a measuring cup or small bowl, whisk together buttermilk, scallions and cayenne. Add to flour mixture, tossing gently with a fork until mixture begins to come together. Gather dough together with hands. Knead 1 or 2 times in the bowl.
Transfer dough to lightly floured surface; roll out dough or pat into 1/2-inch-thick rectangle, 8 x 6 inches. With a floured knife, cut dough into 8 smaller rectangles, about 3 x 2 inches. Transfer pieces of dough to ungreased baking sheet, spacing 1-1/2 inches apart.
Bake in 450 degree F oven for 12 to 15 minutes or until biscuits are puffy and slightly browned on top. Transfer biscuits to wire rack and let cool slightly. Serve warm. Makes 8 biscuits.