To a medium bowl, add the milk and vinegar, and mix. Set aside. You will see it starts to curdle, and this is what creates vegan buttermilk.
Returning to the medium bowl containing the buttermilk, add all of the remaining wet ingredients. Mix thoroughly.
Add all the wet ingredients to the shallow well in the dry ingredients and mix until everything is just combined, but be careful not to overmix.
Divide the cake batter between the two cake pans. Gently tap the pans on the counter a couple of times to level out the batter and to remove any air bubbles. Bake in the centre rack of the preheated oven for 35 minutes, or until a thin wooden skewer inserted into the centre comes out clean.
To a large bowl, add the powdered sugar, cocoa powder, then add the soy milk, and vanilla extract. Start mixing at a low speed and then gradually increase until everything is well combined. Once the melted chocolate has cooled off a bit, mix it in with the other ingredients until fully combined.
Once your cakes are done baking, let them cool off for 10 minutes in the cake pan before transferring to a wire rack to let them cool off completely. Be patient with this, as a warm cake will cause the frosting to melt when applied. If desired, the cakes can be levelled out with a serrated knife to create a flat surface for stacking the cakes on top of each other.
Rip or cut a piece of parchment paper in 6 pieces, of about 10 x 13 cm and place these pieces in a circular shape on your cake stand just wider than the size of your cake. These parchments papers will later be removed and they are used to make the end result neater, as less frosting gets on your cake stand. Place one of the cakes on the cake stand.
Using a spatula or butter knife, spread a layer of frosting on top of the cakes. Place the second cake on top upside down and frost the top and sides with the leftover frosting, then remove the parchment paper squares from under your cake.
Decorate the chocolate cake with desired toppings, and enjoy!