Preheat oven to 350°F. Prepare an 8 inch or 9 inch round cake pan, or an 8x8 square baking pan. (8 inch pan will make a thicker cake!) Spray the inside with cooking spray, then cut a round (or square if using a square pan) piece of parchment paper so that it fits in the bottom of the pan. Set aside.
Add the white vinegar to a liquid measuring cup. Pour milk into the cup until it reaches 1 cup (you’ll have one cup of liquid, total, so you'll need just under 1 cup of vegan milk). Let it sit until it looks curdled (this is your vegan buttermilk!).
Chop apple into small cubes, roughly 0.5”, then measure out 1 ½ cups, and set it aside. Prepare cinnamon streusel by combining butter, sugars, flour, and cinnamon in a small bowl. Mash with a fork/crumble with your hands. Set it aside.
In a large bowl, add flour, brown sugar, white sugar, salt, baking soda, baking powder, and cinnamon. Whisk to combine.
Pour the oil, vegan buttermilk, and vanilla extract into the bowl of the dry ingredients and whisk until a smooth batter forms. Fold the apples into the batter.
Pour the batter into the prepared cake pan, then crumble the cinnamon streusel on top of the batter. Bake for 25-30 minutes, or until a toothpick inserted in the middle of the cake comes out clean. If using an 8 inch cake pan, the cake will take 5-15 minutes longer to bake.
Let the cake cool completely in the pan before removing it. Because it has a light crumb, you have to be careful that it doesn't break! Once cooled, run a butter knife around the outside of the cake a few times. Gently turn the cake upside down onto a plate, then repeat onto another plate so that the streusel is on top. Serve with vegan vanilla ice cream, vegan whipped cream, or enjoy as it!