In a heatproof bowl set over (but not touching) barely simmering water in a pan, stir ¼ cup butter and bittersweet chocolate often until mixture is melted and smooth.
In a large bowl, with a mixer on high speed, beat sugar, eggs, and vanilla until smooth.
Add chocolate mixture and beat until well blended.
Stir in flour and baking powder, then beat just until moistened.
Stir in macadamia nuts, chocolate chips, and coconut.
Drop dough in 1-Tablespoon portions about 2 inches apart on buttered 12×15-inch baking sheets.
Bake clusters in a 350°F regular or convection oven just until firm on the edges but still soft when pressed in the middle, 8 to 10 minutes; if baking two sheets at once in one oven, switch their positions halfway through baking.
Let clusters cool on sheets for 5 minutes, then use a wide spatula to transfer to racks to cool completely.
Take care not to overbake them; their centers should remain soft and chewy.
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