Combine first 4 ingredients in a saucepan; cook over medium heat, stirring constantly, until sugar dissolves. Cover and cook 3 minutes; uncover and cook, without stirring, until candy thermometer registers 236° (soft ball stage). Remove from heat; add peanut butter and vanilla.
Beat at medium speed with an electric mixer until thickened; pour into a buttered 8-inch square pan. Score into 1-inch squares with a knife; cool. Cut into squares.
Microwave chocolate squares and shortening in a 2-cup glass measuring cup at HIGH 1 1/2 minutes or until melted, stirring twice.
Dip fudge squares into chocolate mixture; place on wax paper. Garnish, if desired; let stand until firm.