Preheat the oven to 360°F (180°C). Line a 10 x 14-inch tin with baking paper, leaving a small amount hanging over the edges to make for easy removal.
In a large bowl, mix together the flour, cocoa powder and baking soda.
Stir in the caster sugar.
Add the melted butter, soy milk and vanilla, then mix well until you have a smooth batter.
Transfer the batter to your lined tin and spread it out to the edges. It will be thicker than a regular cake mix, so try to make sure it's spread very evenly.
Tap the tin on the counter a few times to get rid of any large air bubbles, then bake for 12 minutes.
Dust a clean dishcloth/tea towel with unsweetened cocoa powder. You'll need a decent amount, but don't go too crazy or the cake will taste bitter. Do not use baking paper in place of a dishcloth as it will not work.
Let the cake sit at room temperature for 5 minutes so that it's cool enough to handle. Then carefully tip it onto the dishcloth.
Sprinkle a light dusting of cocoa powder onto the cake. Cut off any rough or uneven edges using a knife.
Carefully use the dishcloth to roll it up, with the dishcloth in between the rolled cake.
Let the rolled-up cake sit at room temperature for 90 minutes to cool.