Beat butter in a medium mixing bowl with an electric mixer on medium speed for 30 seconds. Add brown sugar, baking soda, baking powder, and salt; beat till combined. Add sour cream, egg, and vanilla; beat till well combined. Beat in as much flour as you can. Stir in any remaining flour, dates, and walnuts.
Drop by well-rounded teaspoons 5 cm apart onto an ungreased cookie sheet. Bake in a 200ºC oven for 8 to 10 minutes or till light brown. Cool on a wire rack. Drizzle with Powdered sugar Frosting.
Makes 36.
Powdered sugar Icing: Stir together 1.5 cups powdered sugar and enough milk 1-2 tablespoon. to make of drizzling consistency.