Resift flour with soda and salt. Cream half cup butter and brown sugar together thoroughly. Add 1 whole egg and 1 egg yolk and beat well. Stir in vanilla. Add dry ingredients and mix well. Arrange pecan halves in groups of 3 or 5 on greased baking sheets to resemble head and legs of turtle. Roll rounded teaspoons of dough into balls. Dip bottom of each into remaining unbeaten egg white and press lightly onto nuts so tips of nuts show. Bake at 180ºC 10 to 13 minutes. Cool. Meanwhile, to make frosting, combine chocolate, milk and 1 tablespoon butter in top of double boiler. Cook over simmering water, stirring until smooth. Remove from heat and beat in powdered sugar until smooth and glossy, adding more sugar if needed. Frost top of cookies.