1Heat oven to 375°F. Grease 18 regular-size muffin cups or place paper baking cup in each muffin cup. In small bowl, mix all streusel ingredients with fork until mixture resembles coarse crumbs. Set aside.
2In large bowl, mix 2 cups flour, the granulated sugar, baking powder, baking soda and salt. Add sour cream, 1/2 cup butter, the milk, vanilla and egg; stir just until dry particles are moistened. Fold in macadamia nuts and chocolate chips. Fill muffin cups 3/4 full; sprinkle each with 1 1/2 teaspoons streusel.
3Bake 18 to 20 minutes or until toothpick inserted in center comes out clean. Remove from muffin cups immediately. Serve warm.