Preheat oven to 350°F.
Cream butter, shortening and brown sugar until fluffy. Add egg yolk and vanilla. (Set egg white aside in a small bowl.)
Combine flour and salt and add in a little at a time until incorporated.
Divide dough into 20 pieces and roll into balls. Beat egg white in a small bowl. Dip each cookie dough ball in the egg whites and then into the nuts, pressing to adhere.
Place each ball of dough about 2″ apart and using the end of a spoon or your thumb, make an indentation in each cookie. Seal any cracks that form on the sides. Freeze 15-20 minutes.
Bake 16-18 minutes or until set. Remove from the oven and use the back of a 1/2 teaspoon to press the indents again if needed.
Fill indents with jam. Cool completely.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)