Heat oven to 350°F. Line 9-inch square baking pan with foil; grease foil.
Stir together butter, sugar and vanilla in bowl. Add eggs; beat well with spoon. Stir together flour, cocoa, baking powder and salt; gradually add to egg mixture, beating until well blended. Stir in walnuts, if desired. Spread batter evenly in prepared pan.
Bake 20 to 25 minutes or until brownie begins to pull away from sides of pan. Cool completely in pan on wire rack; refrigerate about 2 hours.
Using edges of foil, lift brownie out of pan. Peel off foil; cut into 1-1/4 inch squares and allow to warm up to room temperature.
Line tray with wax paper. Melt small chocolate chips and shortening in small heavy saucepan over very low heat. Dip each brownie square into melted chocolate, covering completely. (Return to heat if chocolate cools and is hard to coat.) Gently tap on side of pan to allow extra chocolate to drip off. Place on prepared tray. Allow chocolate to set at room temperature or refrigerate until set (about 30 minutes); decorate as desired. (Sprinkles should be added just before chocolate sets.) About 32 petit fours.