Mix the topping by combining pecans and brown sugar and mixing well. Add the sour cream and blend; set aside or refrigerate.
Combine flour, baking soda, and salt; set aside. Add sugar to butter in a mixing bowl and cream until light and fluffy; add egg and vanilla and beat well. Gradually add dry ingredients; mix well. Shape into 1 inch balls and place on a cookie sheet, 2 inches apart. Make a depression in the center of each cookie and place 1 teaspoon of topping in the depression.
Bake at 350 degrees for 12 to 14 minutes. Remove to a wire rack and cool.