Hot-Tin-Roof-Sundae

For chocolate-caramel sauce:

Stir first 4 ingredients in heavy medium saucepan over medium heat until sugar dissolves, occasionally brushing down pan sides with wet pastry brush. Increase heat and boil until syrup is deep amber, swirling pan occasionally, about 10 minutes. Turn off heat. Add cream and butter (mixture will bubble up). Stir until any caramel bits dissolve. Stir in salt and vanilla. Add chocolate; stir until melted and smooth. DO AHEAD: Can be made 1 week ahead. Transfer sauce to medium bowl; cover and chill. Rewarm before using.

For ice cream:

Bring milk, 6 tablespoons cocoa nibs, sugar, and corn syrup just to boil in heavy medium saucepan over medium heat, stirring until sugar dissolves. Remove from heat; cover. Steep 20 minutes.

Whisk egg yolks and cornstarch in medium bowl until blended; gradually whisk in warm milk mixture. Return custard to same saucepan. Cook over medium heat, stirring constantly, until custard thickens and just begins to boil, 2 to 3 minutes. Strain custard into large bowl; discard nibs. Whisk in cream and coarse salt. Chill custard until cold.

Process custard in ice cream maker according to manufacturers instructions. Transfer to container; mix in remaining 6 tablespoons cocoa nibs. Cover and freeze at least 6 hours and up to 2 days.

For each sundae, place 2 scoops ice cream in short tumbler; spoon warm sauce over. Sprinkle with Chili-Lime Peanuts.

Bits of shell-roasted cocoa beans; available at many specialty foods stores and from chocosphere.com.