Place ½ cup water and sugar in a large, heavy- bottomed saucepan, and cook over low heat, without stirring, until the sugar is dissolved. Increase the heat to high and boil until the sugar turns a warm mahogany or caramel color, 3 to 5 minutes. Do not stir, but swirl the pan occasionally so the caramel cooks evenly.
Remove the caramel from heat and CAREFULLY pour in the cream and half & half. The caramel will be very hot, and the mixture will bubble up violently, then solidify. Don't worry. Return it to low heat and cook, stirring with a wooden spoon, until the caramel dissolves, 5 to 7 minutes. Add the vanilla. Pour into a container and refrigerate until very cold. (preferably overnight)
Preheat the oven to 350°F. Roast the pecans on a baking sheet for 8 to 10 minutes, until crisp. Cool, chop, mix with the diced chocolate, and store in the freezer until ready to use.
Process the caramel mixture in an ice cream maker according to the manufacturer's directions. When it is frozen, add the cold chopped pecans and chocolate and mix in completely. Transfer to a quart container and store in the freezer until ready to serve.