Place the chopped white chocolate in a medium bowl, and set aside.

In a small saucepan over low heat, heat the heavy cream until it's hot and steaming, but not boiling. About 5-6 minutes.

Pour the cream over the white chocolate and allow to sit for 1 minute, without stirring.

Once the cream has melted the chocolate, stir together until smooth.

In a large bowl, whisk the egg yolks, sugar, and key lime zest until smooth.

Very slowly and gradually, whisk the chocolate mixture into the egg mixture. You don't want to accidentally cook the egg yolks with the heat from the chocolate mixture, so you add the chocolate slowly. Stir until smooth.

Place 4 ramekins or custard cups in an 8x8-inch baking pan.

Pour enough water into the baking pan that it rises to about halfway up the ramekins.

Pour the white chocolate mixture evenly into the ramekins.

Bake at 300 degrees for 40-45 minutes, or until the custard is set at the edges, and slightly jiggly in the center.

Allow ramekins to cool for 20 minutes in the baking pan before removing to cool completely.

Refrigerate for at least 1-2 hours, or until chilled.

Top with whipped cream, and sprinkle with chopped macadamia nuts, if desired, and serve!