Cook chicken until no longer pink in the center by either roasting it or poaching it in water or chicken broth. After it has cooled completely, dice into small pieces.
In a small bowl stir together mayonnaise and sour cream. In a very large bowl stir together chicken, celery, grapes and sour cream mixture. Add salt and pepper to taste. Cover and chill 2 to 24 hours.
Preheat oven to 425 degrees. In a small bowl whisk together milk and egg until blended; set aside. Blend flour, baking powder, salt, sage and pepper in a food processor or large bowl. Add cold butter and pulse or quickly blend with your fingertips until mixture resembles coarse meal. Do not overmix. Add milk mixture and process just until moist clumps begin to form. Transfer dough to a floured surface.
Knead dough until it holds together, about 6 turns. Roll out dough to 1/2-inch thickness. Using a floured 2 1/2-to 3-inch biscuit or cookie cutter, cut out biscuits. Reroll scraps and cut remaining biscuits. Transfer to a large nonstick or lightly greased baking sheet.
Bake until golden brown, 10-12 minutes Let cool.
To serve, split sage biscuits and spoon in chicken salad.