Place a rack in the lower third of the oven and preheat to 350 degrees F.
In a small saucepan over low heat, melt the butter with the chocolate. Remove from the heat and stir in both sugars. Pour into a large mixing bowl and set aside to cool for about 5 minutes.
Add the eggs, rum, and vanilla, stirring just until blended. Stir in the flour and salt, then the coconut, cashews, and cacao nibs.
Spread the batter in a greased 9 x 9-inch baking pan. Bake for 22 minutes. Cool thoroughly before cutting into 16 squares, each about 2 1/4 x 2 1/4 inches.
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