Preheat your oven to 350°F.

In a medium bowl, mix the shortening, butter, sugar, egg yolk, and vanilla until smooth. Mix the sifted flour and salt into the mixture.

In a small bowl, whisk the egg whites until just frothy. Add chopped walnuts to a separate bowl.

Roll the dough into one inch balls. Dip the doughballs into the egg whites and then roll them in the chopped nuts until they are well coated.

Arrange the walnut cookies on a lined cookie sheet. Indent the middle of each cookie gently with your thumb, pressing about half way down. Bake at 350°F for 10 to 12 minutes.

Immediately upon removing the cookies from the oven, carefully press down the centers of the cookies to make a deeper thumbprint. A teaspoon or weathered thumb works well for this.

Allow to cool completely.

Meanwhile, mix together the powdered sugar, vanilla, food dye and just enough whipping cream for a smooth icing. Top each thumbprint with a tablespoon of icing. Store the frosted thumbprint cookies in an airtight container.

Store the cookies in an airtight container for up to 7 days.

Freeze cookies the same day you bake them after they have cooled. Store them in an airtight container and thaw at room temperature.