Preheat oven to 350 degrees.
In a large mixing bowl, mix together the butter and sugar until completely smooth. Then, add in the egg and vanilla and mix some more.
Add the flour and baking soda to the mixing bowl and mix together.
Place rounded cookie dough balls on a cookie sheet.
Bake for 10 minutes or until golden brown.
Remove from oven and place the pastel M&M's on the top of the cookies while they are still warm.