Preheat oven to 350 degrees Fahrenheit. Grease the bottom and sides of an eight-inch square pan with margarine.  Take a piece of aluminum foil larger than the pan and press into the bottom, corners and up the sides of the pan. Grease again. Set aside. 

Place the margarine and chocolate into a medium heavy saucepan over low heat. Cook until melted, mixing often. Remove from heat.

Add the sugar and mix in. Add the eggs, one at a time, and mix well. Add the vanilla, cocoa and salt and mix. Finally, add the potato starch and mix.

Scoop into the prepared pan and bake for 40 minutes. Let cool. Lift the foil and brownie out of the pan and cut into squares. Store in an airtight container at room temperature for up to five days or freeze for up to three months.