Combine the soymilk and vinegar, allowing them about 5 minutes to curdle before use. Pour this mixture into your stand mixer and add in the oil, sugar, and instant coffee. Mix well, beating until slightly frothy. In a separate bowl, sift together the flour, cocoa, baking powder and soda, and salt. Whisk these dry ingredients briefly and add it into the stand mixer in about three additions. Give the mixer enough time to catch up with your demands before pouring in the next load, but don’t let it go for too long either, or else you will overmix and end up with a tough cake. Once the dry goods have been used up, add in the vanilla and mix just long enough to incorporate. Distribute the batter between your prepared pans and use a spatula to smooth down the tops.
Meanwhile, start whipping up the frosting. Start with a very ripe avocado—this is critical, because one that is still a bit too firm will not purée smoothly. Scoop out the insides (sans pit) and purée in your food processor or stand mixer, adding in the lemon juice immediately and pulsing to combine. Don’t worry, you won’t taste this; it will just prevent the frosting from turning brown.
Toss in the margarine and turn on the motor to incorporate yet again. Scrape down the sides of the bowl as needed and make sure that it’s completely homogeneous before proceeding. Slowly add in the sugar in three separate additions, making sure the top is fully covered so that it doesn’t fly out, until the powder is completely mixed in and you’re happy with the thickness and consistency. Add the salt, vanilla, crème de menthe, and peppermint extract. Allow the food processor to run, mixing and whipping the frosting for about 5 minutes until it’s light and fluffy. Frost your cake however you like.
For further adornment, though it’s really gilding the lily at this point, you can also whip up a quick ganache drizzle. Just place both the chocolate and nondairy milk in a microwave-safe dish and heat for 30–60 seconds in the microwave, until the chocolate is mostly melted. Stir thoroughly with a spatula until completely smooth, and voila, instant ganache! Drizzle to your heart’s content.