How to Make It
Line a 9-inch square baking pan with parchment paper, allowing parchment to extend over sides of pan. Grease paper.
Combine heavy cream and powdered sugar in a small saucepan. Bring to a simmer over medium-low, stirring occasionally. (Do not bring to a boil.) Place chopped chocolate in a medium bowl. Pour cream mixture over chocolate; let stand 1 minute. Gently stir mixture until chocolate melts and mixture is smooth. Stir in vanilla.
Combine caramels and evaporated milk in a saucepan over medium-low. Cook, stirring often, until caramels melt and mixture is smooth, 9 to 10 minutes. Remove from heat; let stand 10 minutes.
Place 1 layer of graham cracker sheets in bottom of prepared pan, breaking crackers to fit as needed. Pour one-third of the caramel mixture over crackers. Using an offset spatula, smooth caramel to edges of pan. Pour one-third chocolate mixture over caramel; smooth chocolate to edges of pan. Repeat layers2 times with remaining graham cracker sheets, caramel mixture, and chocolate mixture, ending with chocolate. Sprinkle chopped toffee candy bars over top. Chill, uncovered, 4 hours or overnight.
Lift toffee bar mixture from pan using parchment. Trim edges if necessary. Cut into 16 pieces. Serve chilled.