For the cake:

1. Preheat oven to 350ºF/180ºC. Butter or grease a 8-inch springform cake pan.
2. In a large bowl, sift together flour, cocoa, baking soda and powder, and salt. Mix a little to combine and make a well in the center.
3. Add soy milk, vinegar, sugar, oil and vanilla. Beat or whisk until very well combined. The batter will be quite runny.
4. Pour into prepared pan and bake for for about 50 minutes or until a tester comes out with almost no crumbs attached. It will feel springy to the touch.
5. Let cool on a wire rack and then unmold onto the serving platter.
6. Cover the top with the chocolate mousse and decorate with sliced fresh strawberries. Sprinkle with sugar and serve.

For the chocolate mousse:

1. Begin making the mousse right after the cake is cooled and unmolded. Have ready a large bowl with ice and some cold water, enough so that the bottom of the bowl where you melt the chocolate fits comfortably.
2. Melt chocolate over hot water or in the microwave. Transfer to a large bowl (or use a large glass bowl to melt the chocolate and mix) and add the hot water all at once. Put the bowl over the one with the ice water.
3. With an electric mixer beat at high speed. For a few minutes it will not thicken much, but it will start to do so unexpectedly. And it will thicken stiffly just after that.
4. If the mousse is too thick to spread, add a few teaspoons very hot tap water and beat until smooth. Use immediately.