Preheat oven to 350°F. Grease an 8x8 inch baking pan.
In a medium saucepan, melt together the peanut butter, butter, and brown sugar over medium heat, stirring constantly, until smooth. Remove from heat and stir in the salt, vanilla extract, and baking soda.
Spread the mixture evenly into the prepared pan. Sprinkle the marshmallows over the top and gently press into the mixture.
Bake for 10-12 minutes, or until golden brown. Let cool completely before cutting into bars.
Interesting Facts
Peanut butter was invented in the late 1800s by a Canadian doctor.
Marshmallows were originally made from the sap of the Marsh Mallow plant.
These bars are best stored in an airtight container at room temperature.