Line 2 baking sheets with parchment and toast the pecans.In a heavy saucepan, combine the sugar, brown sugar, cream, milk, butter, and salt.Cook over medium heat, stirring, until the sugar dissolves and the mixture starts to boil.Clip on a candy thermometer and cook, stirring occasionally, until it reaches 236°F to 238°F.Remove from the heat. Stir in the pecans and vanilla.Stir vigorously with a wooden spoon for 2 to 3 minutes, until the mixture thickens, loses some gloss, and turns slightly cloudy.Quickly drop tablespoons onto the parchment, spreading slightly.Let the pralines cool at room temperature until firm, about 30 minutes.