Preheat oven to 350° F.
Beat together 1/2 c. butter, egg yolk, brown sugar, and vanilla in a medium bowl until fluffy.
Beat in flour and salt.
Whisk egg white in a separate, small bowl until frothy.
Form dough into small balls (about 3/4"). Roll in egg white and then in finely chopped walnuts.
Place balls about 2 inches apart on a baking sheet.
insert your index finger into the center of each cookie to form a deep well.
Bake 12-15 minutes until lightly browned.
Remove cookies from sheet and let cool 30 minutes.
Meanwhile, beat together 1/2 c. powdered sugar, 1/4 c. butter, and 1 tbsp. cherry syrup
Beat in remaining powdered sugar and syrup until a soft frosting is formed.
Fill cooled cookies with cherry frosting.