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.
Beat in sugar, then vanilla and eggs, until well combined.
Stir in flour until just combined.
Pour into a well greased or foil lined 11x7 inch or 13x9 inch baking dish. Bake in the preheated oven for 20 minutes (if using the larger pan, adjust cooking time down by a few minutes).
Remove from the oven (brownies will jiggle a little but be done on top) and sprinkle on nuts (optional), half of the chocolate chips and M&Ms, then shredded coconut. Pour sweetened condensed milk evenly over the top. Spread gently to make sure it is evenly distributed. Sprinkle on remaining chocolate chips, M&Ms, nuts and finally the sprinkles. **NOTE - if you use an 13x9” pan like I did, you may need about 1/4 cup more of each of the chips, M&Ms and coconut. I also only used about ¾ of the can of sweetened condensed milk. For a 13x9” pan of brownie bars, use the full can.
Bake for an additional 25 minutes, or until the edges are golden and the middle is set and just starting to brown.
Cool completely before cutting into squares with a sharp knife.
Store in a tightly sealed container for up to one week.