Cupcakes: Put an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Line 36 mini cupcake molds with paper liners. Set aside.
In a large bowl, combine the pumpkin, cake mix, milk, oil, eggs, and pumpkin pie spice. Using an electric hand mixer, beat the mixture on low speed until smooth. Increase the speed to high and beat until light and fluffy, 2 minutes. Using a small ice cream scoop, fill the prepared molds with the batter. Bake until puffed and golden, about 10 to 12 minutes. Cool in the pan for 10 minutes. Transfer to a wire rack to cool completely before frosting, about 20 minutes.
Frosting: In a medium bowl, combine the powdered sugar, cream cheese, butter, cocoa powder, and sour cream. Using a hand mixer, beat on low speed until smooth. Increase the speed to high and beat until light and fluffy, about 1 minute.
Using a small spatula, spread the frosting on top of the cupcakes. Decorate with mini-chocolate chips, sprinkles, colored sugar, and small candies.
The batter can also be spooned into 12 regular-sized cupcake molds and baked in a preheated 350 degree F oven for 20 to 25 minutes.