Preheat the oven to 350°F (175°C).
Place shortbread cookies in a large resealable plastic bag and crush to fine crumbs with a rolling pin, or pulse in a food processor for extra-fine crumbs.
Add the melted butter to the crushed cookies in the bag and mix until the crumbs are evenly coated.
Press the crumb mixture firmly into the bottom of a 9x13-inch pan to form an even crust.
Bake the crust at 350°F for about 10 minutes, until golden; remove and let cool.
In a medium bowl, beat the softened cream cheese with the powdered sugar until smooth. Fold in 2 cups of the whipped topping until combined and spread this cream cheese layer evenly over the cooled crust.
In another bowl, whisk together the whole milk and the two packages of instant pistachio pudding mix until thickened according to package directions, then pour the pudding layer over the cream cheese layer.
Spread the remaining whipped topping over the pudding layer in an even layer.
Refrigerate the dessert until set (chill for at least 2 hours). Before serving, top with M&Ms or chopped pecans if desired.