Preheat oven to 350° F. Line cookie sheet with parchment paper.
In medium bowl, combine flour, baking soda, baking powder and salt. In large bowl, beat shortening and sugar electric mixer until well combined. Add egg, sour cream and vanilla; beat well. Add flour mixture; beat until just incorporated. Stir in nuts. Drop by rounded teaspoons onto prepared sheets, spacing about 2" apart.
Bake 7-8 minutes or until tops of cookies spring back. Cool on sheet 5 minutes; transfer to cooling grid; cool completely before icing.
For icing, in medium microwave-safe bowl, melt chocolate and butter at 50% power until smooth. In small bowl, microwave cream 10-15 seconds. Combine chocolate, cream, and vanilla extract; whisk 2 minutes. Set aside to thicken.
Spread fudge icing on each cookie. Sprinkle with additional chopped walnuts, if desired.