Prep the pans: Preheat your oven to 350°F (180°C). Lightly grease two 8" or 9" round cake pans, then line the bottoms with parchment circles.
Mix the dry ingredients: In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Set aside.
Mix the wet ingredients: In a separate bowl, whisk together the plant-based milk, oil, vinegar, and vanilla.
Combine: Pour the wet ingredients into the dry ingredients and mix until just combined. Some small lumps are totally fine. Overmixing can cause a dense cake so keep a light hand.
Bake: Divide the batter evenly between the prepared pans. Bake for about 40 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool fully in the pans before frosting.
Make the frosting: In a large bowl or stand mixer, beat the vegan butter until light and fluffy. Add the powdered sugar, cocoa powder, and vanilla and continue mixing until smooth. If the frosting is too thick, beat in plant-based milk 1 tablespoon at a time until it’s perfectly fluffy and spreadable.
Frost: Make sure the cakes are completely cool so the frosting doesn’t melt. Run a knife around the edges of the pans, flip the cakes out, and peel off the parchment. Place one cake on a serving plate and spread ⅓ of the buttercream over the top. Add the second cake layer and frost the top (and sides if desired). Chill the cake briefly if your frosting feels soft.
Serve: Store in the fridge or a cool place until serving. The cake will stay fresh for 5–7 days in the fridge.