Preheat oven to 325 degrees F (165 degrees C). Line a 9x11-inch baking pan with parchment paper.
In a saucepan over low heat, melt together the dark chocolate and butter. Beat the mixture with an electric mixer on low speed until well blended, and beat in eggs, sour cream, vanilla extract, sugar, and flour until thoroughly combined. Stir in the macadamia nuts, and pour the batter into the prepared baking pan.
Bake in the preheated oven until the top forms a cracked crust, 30 to 35 minutes. Let cool in the pan about 10 minutes before moving to a wire rack to finish cooling.
