Preheat the oven to 350 degrees F and line a cupcake pan with paper liners.
In a large bowl, add the flour, sugar, cocoa, baking powder, salt, and cinnamon. Whisk until combined.
In a separate bowl, add the milk, pumpkin puree, oil, eggs, and vanilla. Whisk until incorporated.
Add the wet ingredients to the dry and whisk just until combined. Scrape down the edges and bottom of the bowl.
Fill each cupcake liner ¾ of the way full.
Place in the preheated oven and bake for 15-18 minutes or until a toothpick inserted into the center of a cupcake comes out clean.
Allow the cupcakes to cool 5 to 10 minutes in the pan before transferring them to a wire rack and cool completely.
Chocolate Buttercream
Add the butter to a large bowl or stand mixer bowl. Beat on medium speed until light and fluffy, about 2 to 3 minutes with the hand mixer or 1 minute with stand mixer.
With the mixer on low, slowly add the powdered sugar.
Once all of the powdered sugar has been incorporated, add the melted chocolate and vanilla. Beat just until combined. Scrape down the sides and bottom of the bowl.
Turn the mixer to medium high speed and beat until the frosting comes light and fluffy, about 1 minute.
Add the frosting to a piping bag fitted with a Wilton 1M tip.
Pipe tall swirls of frosting on each cupcake.
Place the cupcakes in an airtight container at room temperature.