Preheat oven to 400 degrees. In a bowl, sift together flour, baking powder and salt. Add brown sugar, butter and sweet potatoes. Beat until fluffy. Gently fold in pears. In a separate bowl, combine baking soda and buttermilk. Mix all ingredients together and stir until moist. Turn out on floured surface and kneed lightly. Roll out and cut into biscuits. Bake on baking sheet for 20 minutes. Serve warm with butter or a jam of your choice.