Preheat oven to 350°F. Oil or grease 2 8" cake pans (or 9" pans). Cut out a round of parchment paper to line the bottom of the cake pan. Dust with flour to coat the inside of the pans and tap out any remaining flour that isn't sticking. You want a light coat of flour for the batter to cling to as it rises to ensure an even rise and no "dome" in the center.
Sift together the flour, sugar, baking powder, salt. Double sift for best results.
In a large mixing bowl whip the butter until smooth and no chunks are visible. Scrape the sides and whip again. Mix in the milk, oil, and vanilla.
In a separate bowl use a hand mixer or stand mixer to whip the aquafaba and cream of tartar until soft peaks form. See blog images for an example.
Mix the dry ingredients, not the aquafaba yet, into the wet ingredients. Start with 2 cups of the dry ingredients and mix until smooth. Add the remaining dry ingredients and fold them in until no dry is visible. It will be thick but don't overmix.
Fold the whipped aquafaba into the batter. Don't mix. Fold by scooping the batter from the bottom of the bowl up and then through the aquafaba. The goal is to keep as much of the fluffy aquafaba still fluffy and aerated. This will help your cake stay light.
Divide the batter between the two prepared cake pans. Jiggle the pans a little to spread to the sides.
Bake on the center rack of the oven for 30-35 minutes for 8" pans and 25-30 minutes for 9" pans. An inserted toothpick or cake tester will come out clean. Allow to cook on a wire rack in the cake pans for 15 minutes before removing.
Remove the baked caked by placing a wire rack on top of an individual cake pan and flipping. The cake should come out of the pan. Remove the parchment paper. Use a wire rack on the bottom of the cake and flip again to get the cake with the top up. Repeat with the other pan. Allow to cool to room temperate before frosting or wrapping to freeze.
Frost, decorate, and enjoy!