Lightly grease a 5x9-inch loaf pan using a little of the margarine.
Place the remaining margarine, vegan sugar, cocoa, vanilla and soy milk in a heatproof mixing bowl or the upper part of a double boiler (I use a small pot in a large pot of water).
Place the bowl or boiler over simmering water and stir until smooth. Add nuts if desired.
Pour the mixture quickly into the prepared pan. Chill thoroughly and cut into squares.