Preheat the oven to 350°F/175°C. Make three parchment paper rounds for three 6-inch round cake pans and line each pan and lightly grease the sides to prevent sticking). Alternatively, you can use two 8-inch round pans.
Combine the plant-based milk and apple cider vinegar, stir, and set aside to curdle.
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. I like to sift the mixture together so it’s extra fine and there are no clumps).
In a large bowl, add the softened vegan butter, cane sugar, and brown sugar. Using an electric mixer, beat on medium speed until light and fluffy, about 3 minutes. Add in the vanilla extract and espresso powder and beat to combine.
Switch the mixer to low speed and add in the milk-vinegar mixture until combined. Gradually add half of the dry ingredients and mix on low speed until well combined, about 1 minute, scraping the sides as needed with a spatula. Switch to a wooden spoon and stir in the remaining dry ingredients. The batter will be very thick at this point.
Carefully pour in the boiling water, continuing to mix with a wooden spoon until well combined. Fold in the dark chocolate chunks or chips with a silicone spatula.
Divide the batter equally in the prepared pans, using a scale to measure each pan so each cake is equal in size.
Bake the cakes for 25 minutes until the center is set and the cake has puffed up a bit and just started to pull away from the pan, and a toothpick inserted comes out with only a few moist crumbs. If you are using two 8-inch pans, you’ll want to bake them a bit longer, 30-35 minutes.
Allow the cakes to cool for 10 minutes in the pan, then transfer to a wire rack to cool completely before inverting.
While the cakes are cooling, make the Hibiscus Frosting. With an electric mixer on medium speed, beat the softened vegan butter until fluffy, about 1 minute. Slowly add the powdered sugar in 1/2 cup increments, mixing on low speed until well combined and fluffy, scraping down the sides as needed. Add the vanilla, salt, and hibiscus powder, and beat until smooth. If the frosting is too thick, add the plant-based milk, a tablespoon at a time, until you reach your desired consistency.
To assemble the cake, place one layer cake on a cake platter or plate. Add 1/4 of the frosting and smooth out with an offset spatula. Top with the next layer cake and repeat the process. Using the offset spatula, apply the remaining buttercream to the sides of each cake layer, starting at the bottom.