Heat oven to 350 degrees. Line a 9-inch square baking pan on all sides with two pieces of parchment paper (overlapping) to hang over the sides, buttering as needed to keep them in place.
In large bowl (I like to use an 8-cup batter bowl with pour spout), melt the butter in the microwave for 30 seconds to a minute and stir, then add the cocoa powder and whisk until smooth. Alternately, melt the butter in a 3-quart saucepan over medium-low heat and then add the cocoa powder (using the pan to mix the rest of the batter).
Stir in the sugar until smooth. Add the eggs and vanilla, beating until completely combined. Stir in the flour until smooth.
Pour into the prepared pan and bake until a toothpick comes out with no liquid, only a few crumbs, about 25 minutes. Remove from oven and then reduce heat to 325 degrees.
Make Topping
While the brownies are baking, add all the nuts, chips, and coconut you're using to a medium bowl. Stir in the condensed milk, melted butter and salt until well combined. Set aside while brownies finish baking.
Once brownies are removed from the oven, spoon the topping evenly over the warm brownie base, spreading gently to the edges, making sure the topping doesn't clump in places.
Bake the bars until the topping is golden browned, about 25 to 30 minutes (start checking at the 20 minute mark, though - you want to make sure the center is just starting to brown).
Remove to a wire rack and let the bars cool completely in pan before cutting. Lift the edges of the parchment to remove the bars to a cutting board and use a large knife to cut into 20 (cut 4x5) or 25 (cut 5x5) pieces.
Notes
*To use an unsweetened chocolate bar: omit cocoa powder and 1/4 cup of butter. Melt 3/4 cup butter and 4 ounces of unsweetened chocolate together instead.Storage:
You can keep these bars at room temperature in an airtight container for 2-3 days or in the fridge for 5-7 days.
Freezing: For longer storage, these freeze wonderfully - keep them 4-6 months. If layering in a container, use parchment or wax paper between the layers.