Preheat the oven to 350 degrees Fahrenheit. Line a large cookie sheet with parchment paper or a silicone liner.
In a medium sized bowl, add the butter, brown sugar and granulated sugar, and cream it all together for 1 to 2 minutes, until the mixture is light and creamy.
Stir in the egg yolk, and vanilla extract.
Add in the flour, salt, and baking soda, and stir it all together till you have a soft cookie dough.
Add in the m&ms and chocolate chips, and fold them evenly into the dough.
Place the dough onto the prepared cookie sheet and press it out into a large heart, about 1/2 to 3/4 inch thick. Exaggerate the inner and bottom v of the heart since the cookie will grow a bit as it bakes.
Bake the cookie until the edges are golden and set, and the center looks barely cooked. (You'll want to watch it closely since the cooking time can vary greatly based on the thickness of your dough) Mine have been done in 10 minutes every time.
Top the cookie with additional m&ms, chocolate chips, and now sprinkles if desired, for a cuter appearance.
Let the cookie cool for about 5 minutes on the cookie sheet before digging in, or transferring to a cooling rack to cool completely.