These gooey bars will be your new favorite no-bake cookie. Serve chilled for less mess.
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 layers 2 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.