Preheat the oven to 350°. Prepare an 8 inch cast-iron skillet with parchment paper.
In a small bowl, whisk together the flour, baking powder, and salt.
In a mixing bowl, beat together the melted butter and sugars. Add the egg and vanilla, beating until well blended.
Add the flour to the sugar mixture and mix until combined.
To the batter, stir in ¾ cup of the candies.
Press the dough into the prepared skillet.
Bake the cookie for 15 to 20 minutes or until the edges are lightly golden brown and the top of the cookie is set. Place the skillet on a wire rack and let it cool for 10 minutes.
To melt the chocolate, place the milk chocolate morsels into a microwave-safe bowl. Heat on high for 30 seconds. Stir well and repeat for 15 seconds, stirring after each heating cycle, until the chocolate is melted.
Spread the melted chocolate over the cookie and press the remaining M&Ms into the chocolate.
Let the cookie pizza rest for 15 to 20 minutes or until the cookie is completely cooled and the chocolate is set.