In a medium mixing bowl, beat together sugar, butter, and salt until light and fluffy. Separate the egg, reserving the white. Add the yolk, melted chocolate and vanilla to butter mixture and beat well.
Add flour. Beat or stir until combined. Roll dough into 1-inch balls. Mix egg white with water. Dip balls of dough into egg white and then the nuts.
Place on greased cookie sheet. Press centers with your thumb. Bake in a 350 degree F oven for 10 to 12 minutes or until edges are set. Cool 1 minute on cookie sheet. Remove to wire rack to cool.
Prepare the Thumbprint Frosting. Fill cookie centers with a small spoonful of frosting. Makes 36 cookies.
For a pipable cookie filling, use a buttercream frosting recipe. Or spoon jam into the centers of the cookies.
Combine powdered sugar and enough milk to make a spooning consistency. Tint with a few drops of food coloring, if you like.