Preheat oven to 350°F. Line a 9-inch square baking pan with greased foil or parchment paper.
In the bowl of a stand mixer fitted with a paddle attachment, or in a large bowl with a hand-held mixer, beat the butter, brown sugar, and 1/3 cup granulated sugar on high speed until light and creamy, about 2-3 minutes.
Add the eggs one at a time, mixing after each and scraping the bowl down after each addition.
Add the flour, cream of tartar, baking soda, salt, and 1/2 teaspoon cinnamon. Mix until just combined.
Add the white chocolate chips and macadamia nuts and mix until combined.
Spread the batter into the prepared baking pan. Combine the remaining 2 tablespoons of sugar and 1/2 teaspoon cinnamon and sprinkle on top.
Bake until edges are golden and center is set, about 25-30 minutes. Let cool to room temperature.
Drizzle with melted chocolate and macadamia nuts, if desired. Chill in the fridge 15 minutes to set, then slice and serve.
Calories: 381kcal, Carbohydrates: 46g, Protein: 5g, Fat: 21g, Saturated Fat: 10g, Cholesterol: 47mg, Sodium: 135mg, Potassium: 188mg, Fiber: 1g, Sugar: 29g, Vitamin A: 299IU, Vitamin C: 1mg, Calcium: 59mg, Iron: 2mg