Preheat oven to 350°F (180°C). Lightly grease two 9-inch (or 8-inch) round cake pans and line the bottoms with parchment for easy release.
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 the plant-based milk, oil, lemon juice (or apple cider vinegar), and vanilla.
Combine: Pour the wet ingredients into the dry. Stir until just combined. Don't overmix, some lumps are fine. Overmixing can lead to dense or flat cake layers. A light hand = fluffy cake!
Bake: Divide batter evenly between the pans. Bake for 22–28 minutes, or until the tops are lightly golden and a toothpick inserted into the center comes out clean. Let the cakes cool completely in the pans on a wire rack.
Make the frosting: In a large bowl or stand mixer, beat the vegan butter until light and fluffy. Add the powdered sugar, vanilla, and food coloring (if using), 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 & decorate. Ensure your cakes are cooled completely so the frosting doesn't melt. Spread frosting between the layers and over the sides and top. Add sprinkles or other decorations if desired.