Preheat the oven to 350°F (175°C). Line a mini cupcake pan with 24 cupcake liners, or one regular muffin pan with 10 cupcake liners.
In a medium bowl, whisk together flour, cinnamon, baking powder, baking soda and salt. Set aside.
In a mixing bowl with paddle attachment, beat the butter and sugar until fluffy for 1-2 minutes on a medium high speed.
Add egg, pumpkin puree and vanilla extract and beat for another 2 minutes, or until well combined. and mix until just combined. Don’t overmix to avoid dense and heavy cupcakes.
Add half of the flour mixture and mix on low speed until just combined. Then stir in buttermilk, and finally add the remaining flour mixture. Mix the batter just until combined.
Divide the batter into prepared liners 2/3 way up. (TIP: I like using this medium cookie scoop. 1 scoop per mini cupcake, or 3 scoops per regular cupcake.)
Bake for 10-12 minutes. To test the doneness, insert a toothick in the center of a cupcake. If it comes out clean, it ready. If it comes out with a little bit wet batter, bake for a minute or so more.
Cool the cupcakes in the pan for 5 minutes. Then transfer them to a wire rack to cool completely.