Prep the pans: Preheat the oven to 350°F (180°C). Lightly grease two 9 inch round cake pans (or 8 inch for thicker layers) and line the bottoms with parchment paper.
Mix the dry ingredients: In a large bowl whisk together the flour, sugar, baking soda, and salt.
Mix the wet ingredients: In a medium bowl whisk together the plant milk, oil, lemon juice (or apple cider vinegar), and vanilla extract.
Combine: Pour the wet ingredients into the dry ingredients and stir until just combined. Some small lumps are fine. Avoid overmixing as this can make the cake dense.
Bake: Divide evenly between the prepared pans. Bake for 22 to 28 minutes, until lightly golden and a toothpick inserted in the center comes out clean.
Cool: Let the cakes cool completely in the pans before frosting to keep them moist and easy to handle.
Make the frosting: In a large bowl or stand mixer, beat the vegan butter until light and fluffy. Add the powdered sugar and vanilla and continue mixing until smooth. Add plant-based milk, 1 tablespoon at a time, until the frosting is light, fluffy, and spreadable.
Frost and decorate: Spread frosting between the layers and over the top and sides. Decorate as desired.