Preheat oven to 350 degrees F. Line a 15x10x1-inch baking pan with foil, extending foil over the pan edges. Lightly grease foil. Set aside. In a very large bowl, combine cake mix, 1/3 cup melted butter, and egg. Beat with an electric mixer on low speed for 1 to 2 minutes or until mixture is combined. Press mixture onto bottom of the prepared pan. Bake in the preheated oven for 12 minutes. Cool completely in pan on a wire rack.

In a large bowl, combine 3/4 cup softened butter, peanut butter, and vanilla. Beat with an electric mixer on medium speed for 30 seconds. Add powdered sugar and beat until combined, scraping bowl occasionally. Beat in the 2 tablespoons whipping cream. Stir in the chopped peanuts. Spread mixture evenly over the cooled crust. Place bars in the refrigerator while preparing the ganache.

In a small saucepan, bring 1/2 cup whipping cream just to boiling over medium-high heat. Remove from heat. Add chocolate pieces. Do not stir. Let stand 5 minutes. Stir until smooth. Cool 15 minutes. Very gently pour ganache over bars, lightly spreading it to the edges of the bars with a spatula. Cover and chill for 1 to 2 hours or until set. Using the edges of the foil, lift the uncut bars out of the pan. Cut into bars. Makes 48 bars.

Place bars in a single layer in an airtight container; cover. Store in the refrigerator up to 3 days or freeze for up to 3 months. To serve, thaw bars, if frozen. Let refrigerated bars stand at room temperature 15 minutes before serving.