In a sifter combine the flour, baking soda and salt. Sift onto a large piece of waxed paper and set aside until needed.
Heat 1-inch of water in the bottom half of a double boiler over medium heat. With the heat on, place the semisweet chocolate in the top half of the double boiler. Use a rubber spatula to stir the chocolate until completely melted and smooth, about 4 to 5 minutes. Transfer to a 1-quart bowl and set aside.
Place the granulated sugar, light brown sugar and butter in a 7-quart bowl. Use a stiff rubber spatula (or a wooden spoon) to cream the ingredients together until smooth. Add the eggs and vanilla extract and mix to incorporate. Add the melted chocolate and mix until combined. Add the sifted dry ingredients and thoroughly combine. Add the M&M's, mixing to incorporate.
Using 3 heaping tablespoons of dough for each cookie (approximately 3 ounces), portion 6 cookies, evenly spaced, onto each of 3 nonstick baking sheets (this is a hefty cookie, so don't crowd). Place the baking sheets on the top and center racks of the preheated oven and bake for 14 minutes. Remove the cookies from the oven and cool to room temperature on the baking sheets, about 30 minutes. Store the cooled cookies in a tightly sealed plastic container until ready to serve.