In the bowl of a stand mixer fitted with a paddle attachment, beat the margarine and sugars on medium-high speed until fluffy and well combined, about 3 minutes, stopping as needed to scrape down the sides of the bowl. With the mixer on medium speed, add the melted chocolate and vanilla and beat until just combined. Scrape down the sides of the bowl. Reduce the speed to low, add a third of the flour mixture, and mix until just incorporated. Add a third of the soy milk mixture and mix until just incorporated. Continue with the remaining flour and soy milk mixtures, alternating between each and stopping to scrape down the sides of the bowl as needed, until all ingredients are just incorporated.