Peanut Butter Cookie Pie

1/2 cup unsalted butter, melted
1 large egg
1 cup light brown sugar, packed
1 tbsp. vanilla extract
heaping 1/3 cup creamy peanut butter
1 cup all-purpose flour
pinch salt, optional and to taste
12 to 14 mini peanut butter cups
about 2/3 cup M&Ms (peanut or plain)

Preheat oven to 350 degrees.  Spray a 9-inch pie dish with floured cooking spray; set aside. 
In a large, microwave-safe bowl melt butter, about 1 minute.  Wait momentarily before adding the egg so you don't scramble it.  Add the egg, brown sugar, peanut butter, vanilla, and whisk until smooth.  Add flour, salt, and stir until just combined; don't overmix.  Pour the dough into prepared pie dish, smoothing the top lightly with a spatula.  Evenly insert the peanut butter cups into the dough, pushing them down so the top is almost flush with the surface of the dough.  Evenly insert the M&Ms into the dough, pushing them down.
Bake for about 28 to 30 minutes, or until a toothpick inserted in the center comes out mostly clean.  If your pie has risen in the center, it will deflate as it cools.  Allow pie to cool in pan for at least 30 minutes before slicing and serving; however, for optimal results, cool overnight because it gives the flavors a chance to marry and the pie to really set up fully..  Pie will keep airtight at room temperature for up to 1 week.