Preheat the oven to 325 degrees F Line the muffin tin with cupcakes liners. Set aside.
In the bowl of an electric stand mixer with the paddle attachment, beat butter and oil together on high speed until completely unified, light and creamy - approximately 4-5 minutes.
Sift together cake flour, baking powder, baking soda, and cinnamon.
Scrape down the sides and bottom of the bowl. With the mixer on low speed, gradually pour in sugars. Turn mixer up to high speed and mix until light and fluffy - approximately 2 minutes. Scrape down the sides and bottom of the bowl with a rubber spatula.
With the mixer on low speed, add in the eggs one at a time followed by the vanilla extract. Beat on medium until combined. Scrape down the sides and bottom of the bowl with a rubber spatula.
With the mixer on low speed, add in ⅓rd of the flour mixture. Mix until just combined. Scrape down the sides of the bowl. With the mixture on low speed, add in ½ the buttermilk. Mix until just combined. Repeat. Begin and end with the flour mixture.
Spoon ⅓rd of the cupcake batter into each liner. Next spoon about a teaspoon of cinnamon sugar on top. Fill the liners ⅔rds full and then use a knife to swirl the cinnamon sugar around the batter. Bake for 16-18 minutes or until a toothpick inserted into the centre of the cupcake comes out clean.Allow cupcakes to cool completely before frosting.