For the caramel, mix 1/3 cup water and the sugar in a medium heavy-bottomed saucepan. Cook, without stirring, over low heat for 5 to 10 minutes, until the sugar dissolves. Increase the heat to medium and boil uncovered until the sugar turns a warm chestnut brown (about 350 degrees on a candy thermometer), 5 to 7 minutes, gently swirling the pan to stir the mixture. Be careful; the mixture is extremely hot! Watch the mixture constantly at the end, as it will go from caramel to burnt very quickly. Turn off the heat. Stand back to avoid splattering and slowly add the cream and vanilla. The cream will bubble violently and the caramel will solidify; don't worry. Simmer over low heat, stirring constantly, until the caramel dissolves and the sauce is smooth, about 2 minutes. Allow to cool to room temperature, at least 4 hours. It will thicken as it sits.
To assemble the sundaes, place one scoop of vanilla ice cream and one scoop of butter pecan ice cream in each dish and sprinkle with toasted pecans. Drizzle with caramel sauce and serve extra sauce on the side.
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