Combine the sugar, brown sugar, milk, syrup, butter, salt, and peanut butter in a heavy saucepan over medium heat. Stir until the sugar dissolves. Continue to cook, stirring, until the mixture begins to bubble around the edges of the pan, about 12 minutes. Add the pecans and continue to stir until the mixture thickens, about 8 minutes, and reaches 234º 240ºF. on a candy thermometer, or the soft ball stage, that is, when a bit dropped into cold water forms a soft ball that flattens.
Remove from the heat and drop by the spoonful onto wax paper. Let cool. Lift the pralines off the paper with a thin knife.
Pralines can be stored in an airtight container at room temperature for up to 2 weeks.
Yield: about 1 1/2 dozen