Make the Frosting: In a large bowl (using a hand mixer) or in the bowl of a stand mixer, combine butter, 4 cups of the confectioner's sugar, milk and vanilla. Beat on low, very slowly, gradually increasing the speed as it combines so the sugar doesn't turn into a dust cloud. Beat until light and fluffy, pausing to scrape down the sides of the bowl as needed, about 2-3 minutes.
Check the frosting: Is it about as thick as peanut butter or jam? Great. If it's too runny, add the remaining confectioner's sugar 2 tablespoons at a time, until you reach the desired consistency. (If it's too thick, you can add a little milk, 2 tablespoons at a time.)
Put 2/3 of the frosting in one bowl, and 1/3 in a second bowl. Add 2 drops of blue food coloring to each bowl. Stir to combine, and you should have one light blue frosting (large bowl) and one darker blue (small bowl). You may want to add a drop or two to the smaller bowl to darken the color, or even add half a drop of black to make it an inkier, grayish-blue.
Decorate the Cupcakes: Place a thin layer of light blue frosting to cover the cupcake. Cut the Oreo Thins in half, so they form two semicircles. Gently place them on top of the cupcake, forming the alien's ears. Cover the Oreo Thins and the rest of the cupcake with a little more light blue frosting.
Place the darker blue frosting in resealable plastic bags (like Ziploc) and snip off one corner. Use that to easily outline the alien's ears and top half of his face, leaving two large oval shapes for the eyes. Place a Peanut M&M slightly diagonal in each eye hole. Use black frosting to form a thin smirk.
