Make the cake: Preheat the oven to 350°F. Prepare a bundt pan by greasing well with vegan butter, then coating with a bit of flour or cocoa powder.
In a medium mixing bowl, whisk together the soy milk with the vinegar or lemon juice (this is your vegan buttermilk). Set aside for at least 10 minutes. In a separate, large mixing bowl or the bowl of your stand mixer fitted with a paddle attachment, add the flour, cocoa powder, sugar, baking soda, baking powder, salt, and espresso powder. Mix until combined. To your buttermilk, add the vegetable oil, sour cream, and vanilla. Whisk to combine. Add the wet mixture to the dry and mix until just barely combined. Add the warm water or coffee to the mixture, and mix until just combined. Do not overmix. Scrape the sides as necessary.
Pour your cake batter into the prepared pan. Bake in the preheated oven for 38-42 minutes, until a toothpick inserted into the center comes out clean or with just crumbs. Allow to cool for about 30 minutes in the pan.
Make the chocolate glaze: Add all ingredients, minus vanilla extract, to a saucepan over medium heat. Whisk often until the mixture is melted and smooth. Remove from heat and whisk in the vanilla. Set aside to cool slightly.
Assemble: Flip the cake onto a serving platter. Drizzle warm chocolate glaze all over the cake, allowing any extra to go into the center of the cake.
Serve: Slice and serve! This cake is delicious served warm (especially with a scoop of vegan ice cream!), but can be served at room temperature as well.