Preheat oven to 350°F. Line a 24 cupcake tin pan with liners (or two 12-cupcake pans). Using a stand alone or hand held electric mixer, beat together the cake mix, pudding mix, eggs, oil, milk and sour cream. Beat until well combined, about 2 minutes. Scoop the batter evenly in all 24 cupcake liners. Bake for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean. Remove the cupcakes from the tin and let cool.
While the cupcakes are baking, prepare the mousse by melting the milk chocolate chips in the microwave. Melt for 30 seconds, remove and stir. Place back in the microwave for 30 more seconds and stir. The chocolate chips should be completely melted. If not, continue to melt in 30 second increments. Set aside to cool. Whip the heavy cream with a stand alone or electric handheld mixer until it looks like whipped cream and stiff peaks form. Fold in melted chocolate chips into whipped cream until it is all evenly combined.
To fill the cupcakes, transfer batches into a piping bag. Once the cupcakes have cooled, press the tip of the piping bag into the center of the cupcake (about 1-inch deep) and squeeze while pulling the tip out. (You may certainly skip this step and simply frost the cupcakes with the mousse.)
Using a knife, frost the cupcakes with the remaining mousse and sprinkle with chopped Snickers pieces.