Preheat the oven to 350 F. Line an 8-inch cast-iron skillet with parchment paper.
In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large mixing bowl, use a hand mixer, stand mixer fitted with the paddle attacment, or a wooden spoon to mix together the melted butter and sugars. Add the egg and vanilla, mixing until well blended.
Stir the dry ingredients to the sugar mixture until just combined.
Fold in ¾ cup of the M&Ms.
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 and smooth.
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.
Notes
No cast iron skillet? You can use an 8-inch springform pan or cake pan instead.Store for up to 3 days at room temperature.Recipe adapted from the M&M website.