In a large skillet over high heat with a heat proof spatula, stir together the sugar, corn syrup, and 1⁄4 cup water and bring the mixture to a full boil. Stir in the macadamia nuts. Continue to cook for 5 minutes, stirring occasionally, until the syrup becomes thick and honey colored and you begin to smell the macadamia nuts cooking (THIS IS A KEY STEP, TRUST YOUR NOSE! You’ll be able to smell a slight roasting smell). Remove the skillet from the heat and stir in the baking soda. Working quickly, stir in the butter, cinnamon, and cayenne and continue to stir until the butter has melted. Turn the brittle out onto the prepared baking sheet and, working quickly, use the back of your spatula to spread the mixture out. Don’t worry about any shape, it will harden quickly, just push it out so it’s all one thickness.  Let the brittle cool completely, for about 20 minutes, then break into shards. Store the shards in an airtight container until ready to decorate the cake (shards will keep for 10 days).