Preheat the oven to 350°F (180°C). Grease or line a 9 x 13 inch (23 x 33 cm) baking pan with parchment paper.
Add all of the dry ingredients and whisk until very well combined and there are no lumps.
Add all the wet ingredients and stir until just combined. Be sure to not overmix. Some lumps in your cake batter are fine, as long as they aren't lumps of dry flour.
Pour the batter into your prepared pan and smooth the top with a spatula or spoon.
Bake according to standard cake baking time until a toothpick inserted in the center comes out clean.
Prepare buttercream frosting by beating vegan butter with powdered sugar, vanilla extract, and dairy-free milk until smooth.
Frost the cooled cake and decorate with caramelized apples if desired.
