Lightly coat a 5" x 9" loaf pan using the margarine.
Bring the soy milk and margarine to a simmer over low heat (in a double boiler or in the microwave). Stir frequently. Add the powdered sugar, cocoa, and vanilla and stir until smooth. Add nuts if desired. Remove from heat. (If using a microwave you may heat several times - if necessary - 15 seconds on high between stirring.)
Pour the mixture quickly into the prepared pan. Chill for 1 hour then cut into squares. Makes about 2 to 3 dozen squares.