Hibiscus Thumbprint Cookies

½ cup butter
¼ cup brown sugar (lightly packed)
1 egg, separated
½ t. vanilla
1 cup flour
¼ t. salt
½ cup chopped nuts (optional)
Hibiscus preserves

Heat oven to 350 degrees. Mix butter, brown sugar, egg yolk and vanilla. Mix in flour and salt. Shape into 1-inch balls and place 1 inch apart on ungreased cookie sheet.
If using nuts - beat egg white slightly. Dip each ball into egg white and then roll in nuts. Press thumb deeply in center of each cookie. Bake until light brown, about 10 minutes. Immediately remove from cookie sheet and allow to cool. Fill with hibiscus preserves.