Preheat oven to 350°F. Grease the interior of an 8 inch x 8 inch baking pan. Then line it with a large sheet of parchment paper. The parchment paper should be large enough to line the bottom and the sides. Don't worry if the parchment paper doesn't completely stick to the sides of the pan. Once the batter is finished mixing, the weight of the batter will help the parchment paper stick to the pan.
Add flour, sugar, cinnamon, baking soda, salt directly into baking pan. Whisk together until evenly combined.
Make two small wells and 1 large well in the dry ingredients. Add vanilla to one small well and vinegar to the other small well. Add oil to the large well. Pour the water over the top and whisk everything until batter becomes smooth and evenly combined. Make sure to scrape all the flour from the bottom and the corners of the cake pan as flour tends to get stuck in those areas and you need to make sure all the flour is incorporated.
Place cake into oven and bake for about 35-40 minutes (mine was done around 38 minutes) or until a toothpick inserted comes out clean and the top of the cake should bounce back when you apply light pressure.
While cake is cooking, making the topping. Add cinnamon and sugar to a small bowl and whisk to evenly combine.
When cake is done, sprinkle cinnamon sugar evenly over the surface of the cake while the cake is still warm.
Let cake cool completely before cutting and serving. You can serve as is, or top with frosting or ice cream.
Notes
Make sure you mix in all the flour that may get stuck in the corners of the pan and at the bottom of the pan.
I do not recommend using white sugar unless you are okay with your cake coming out a light reddish brown color.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.