Heat oven to 350°F.
Beat together shortening and brown sugar in large bowl with an electric mixer at medium-high speed until pale and fluffy. Beat in egg, vanilla and peanut butter, then add flour and salt. Mix at low speed until just combined.
Spray a 15 1/2 x 10 1/2 x 1 inch jelly roll pan with no-stick cooking spray. Spread batter evenly in prepared pan and bake 20 to 25 minutes, until top puffs slightly, is firm to the touch and edges pull away from sides of pan. Cool cookie base on rack for 10 minutes.
Melt chocolate chips in large microwave-safe bowl. Alternate heating and stirring frequently until chocolate is smooth and melted.
Remove lid from hot fudge topping and heat according to package directions. Mix warmed hot fudge topping into warmed chocolate. Spread over cookie base and immediately sprinkle with walnuts. Chill in pan, then cut into 2-inch diamonds. Store in an airtight container.
TIP *To toast nuts: Place chopped walnuts in a dry nonstick skillet; cook over medium heat until lightly browned.