For the icing:
In medium saucepan, cook 1/2 cup heavy cream, chocolate, and corn syrup over medium-low heat, whisking constantly, until smooth. Transfer mixture to medium bowl and refrigerate until thickened but still spreadable, about 2 hours, stirring occasionally. (Alternatively, refrigerate mixture overnight until set. Let sit at room temperature until softened and spreadable, about 1 hour.)
For the caramel:
Finely chop 1-1/2 cups pecans. Heat caramels, remaining cream, and salt in saucepan over medium-low heat, whisking constantly, until smooth. Reserve 3/4 cup of the caramel mixture for topping. Stir chopped pecans into remaining caramel mixture.
Place 1 cake layer on cake serving platter. Spread half of pecan-caramel mixture in even layer over top, leaving 1/2-inch border around edge. Place second cake layer on top and repeat with remaining pecan-caramel mixture. Top with third cake layer. Spread chocolate icing over top and sides of cake and refrigerate until set, about 30 minutes. Spread reserved caramel mixture over top of cake, allowing caramel to drip over sides. Garnish with remaining pecans. Slice and serve.