Preheat oven to 350°F. Line a muffin baking pan with 24 paper liners and set it aside.

Place the cake mix, milk, melted butter, eggs, vanilla, and cinnamon in a large mixing bowl. Blend with an electric mixer until well combined. Stop the machine and scrape down the sides of the bowl with a rubber spatula. Increase mixer speed to medium and beat 2 more minutes more, scraping the sides down again, if needed. The batter should look well combined. Divide the batter evenly between the muffin cups. Place the pans in the oven side by side.

Bake the cupcakes until they are golden brown and spring back when lightly pressed with your finger, about 15-20 minutes. Allow the cupcakes to cool in the pan for 10 minutes, then remove them to wire racks to cool completely, about 30 minutes.

Meanwhile, prepare the frosting by placing the butter in a large mixing bowl. Blend with an electric mixer on low speed until fluffy, 30 seconds. Stop the machine and add the powdered sugar, 3 tbsp. milk, vanilla, and cinnamon. Blend with the mixer on low until the sugar is incorporated, about 1 minute. Increase the speed to medium and beat until light and fluffy, 1 minute more. Blend in up to 1 tablespoon milk if the frosting seems stiff.

Spread or pipe the frosting on top of the cupcakes.