Pour puffcorn into a large, greased roasting pan.

Put butter, peanut butter, brown sugar and light corn syrup in a large saucepan (mixture will bubble up so it has to be big).

Bring to a boil and cook for 1 minute, stirring constantly.

Add baking soda and mixture will foam up.

Pour mixture over puffcorn and stir to coat.

Bake at 250 degrees for 45 mintues, stirring every 10 minutes.

Remove from oven, pour onto waxed paper and break into pieces.