Add the cake mix, soda, buttermilk, eggs, vanilla, and cinnamon together in a bowl. Mix on medium speed with a whisk attachment until fully combined.
Pour in a greased and floured 9x13 pan. Bake in the oven at 325 degrees for 35 minutes or until a toothpick comes out clean. Do not open the oven door before at least the 30 minute time mark so the cake doesn't fall in the middle.
Frost with your choice of buttercream and store in an airtight container in the fridge. If storing in the freezer, wrap in plastic wrap and place in an airtight container.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of unfrosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**