Cream butter or margarine, gradually add brown sugar, beating at medium speed of an electric mixer until light and fluffy.
Add 1 egg, 1 egg yolk, and vanilla; beating until well blended.
Combine flour, baking soda and salt; add to creamed mixture, mixing well.
Chill dough at least 1 hour.
Arrange pecan halves in groups of 5 on ungreased cookie sheets, resembling head and legs of turtles.
Shape dough into 1-inch balls, and dip bottoms in remaining egg white. Press gently onto pecans to resemble turtle bodies.
Bake at 350 degrees F (180 degrees C) for 10 to 12 minutes. Cool on wire racks.
Spread frosting on top of cookies.
Combine chocolate squares, milk and 1 tablespoon butter or margarine in a small saucepan; cook over low heat, stirring constantly, until chocolate melts.
Remove from heat.
Add approximately 1 3/4 cups confectioner's sugar; beat until smooth.