Preheat the oven to 350°F (180°C) and line a cupcake tray with 12 cupcake liners.
Sift all purpose flour into a mixing bowl and add white granulated sugar, baking soda, salt and cinnamon. Mix together.
Prepare your homemade buttermilk by adding 1 tablespoon of lemon juice (or distilled white vinegar) to a measuring jug. Add room temperature milk up to the 1 cup (240ml) line. Let it sit for a minute to thicken and curdle.
Add the buttermilk, canola oil, egg and vanilla extract and mix into a batter. Don't overmix.
Divide the batter evenly between the cupcake liners.
Bake for 20 minutes or until a toothpick inserted into one of the cupcakes comes out clean.
Transfer the cupcakes to a wire cooling rack to cool completely before frosting.
Add softened butter to the bowl of your stand mixer. Start off at slow speed, gradually increasing speed and let it whip until your butter is smooth and pale in color.
Add powdered sugar, vanilla extract and milk and beat for a few minutes until fluffy and smooth.
Add the cinnamon last and mix in on low speed until just mixed.
When the cupcakes are completely cooled, pipe on the frosting.