To prepare topping, combine 2/3 cup brown sugar, half cup flour, four tablespoons butter, white chocolate chips, and macadamia nuts. Mix until crumbly; set aside. Preheat oven to 350. Prepare a a 9"x13" pan with cooking spray; set aside. In a mixing bowl, combine remaining butter, granulated sugar, remaining brown sugar, remaining flour, baking powder, and cinnamon. Mix well. Gradually add 1 1/4 cups milk, egg, and pure vanilla extract. Pour half the batter into prepared pan; top with half the topping. Gently spread remaining batter over topping. Sprinkle remaining topping evenly over top. Bake for 30 minutes, or until toothpick inserted in center comes out clean. Cool. Meanwhile, to prepare drizzle, combine powdered sugar, remaining milk, and clear vanilla extract. Drizzle evenly over top cooled cake.