Turn your oven up to 300°F and place a greased jellyroll pan on the bottom rack while you prepare the brittle. Turn the oven off when the pan is hot to the touch; this will allow the brittle to spread nicely.
In a medium saucepan, combine sugar, corn syrup, salt, water, and molasses if using.
Bring to a boil, stirring to combine.
Once sugar has dissolved, stir in peanuts.
Stir until mixture reaches hard crack stage (300-310° F; a drop of it in a cup of ice water will be brittle and break easily, without bending).
Turn off heat, stir in margarine and soda and vanilla (if using), and pour onto prepared pan.
Gently tilt pan from side to side until candy has spread to desired thinness.
Allow to cool completely.
Break into pieces.