1) For the caramel, mix 75ml of 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.
2) Increase the heat to medium and boil the mixture uncovered until the sugar turns a warm chestnut brown (about 180°C on a 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.
3) Meanwhile, heat the cream and the vanilla in a small saucepan over low heat until it starts to steam, do not boil. Once the caramel is dark, slowly add the warm cream and vanilla mixture. 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.
4) 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 the toasted pecans. Drizzle with the caramel sauce and serve extra sauce on the side.