Preheat oven to 350 degrees F. Line a 9-inch baking pan with foil or parchment paper and spray with nonstick cooking spray.
Place butter and chocolate chips in a large, microwaveable mixing bowl. Microwave on high for 30 seconds. Stir, and continue microwaving and stirring in 30 second intervals until chocolate is melted and smooth.
Beat in sugar, then vanilla and eggs, until well combined.
Stir in flour until just combined. Pour into the prepared baking dish.
Bake in the preheated oven for 20 minutes.
Remove from the oven (brownies will jiggle a little but be done on top). Layer as follows: nuts, half of the chocolate chips, half of the M&Ms, and finally all of the shredded coconut. Pour sweetened condensed milk evenly over the top and spread gently to make sure it is evenly distributed. Sprinkle on remaining chocolate chips, M&Ms, and finally the sprinkles.
Bake for an additional 25 minutes, or until the edges are golden and the middle is set and just starting to brown. If the top if browning too quickly, tent loosely with foil.
Cool completely before cutting into squares with a sharp knife.
Store in a tightly sealed container for up to one week.
Refrigerate: In an airtight container at room temperature for up to a week. They can be stored in the refrigerator for up to 2 weeks. For best results, place parchment paper between layers to keep them from sticking.Freeze: Lift brownies out of the pan and wrap with plastic wrap and then aluminum foil. Freeze for up to 3 months. Or wrap up individual squares for a decadent treat any time you like. Thaw on the counter for an hour before eating.