In a clean bowl, whisk together the 2/3 cup of flour, 1/2 cup of sugar, 1/2 teaspoon cinnamon, 1/4 teaspoons salt and orange zest.
Add the four tablespoons of melted butter and stir until well mixed and crumbly.
Keep in the refrigerator until ready to use.
Coffee Cake Muffins
Preheat oven to 425°F and line a 12-cup standard muffin pan with paper liners.
Whisk together the flour, brown sugar, baking powder, baking soda, and salt in a large bowl. Combine the orange juice, sour cream, melted butter, egg, and vanilla in a medium bowl. Add sour cream mixture to flour mixture and fold in just until incorporated – the muffin batter will be thick.
Using a muffin scoop, fill each paper liner with about 1/4 cup of batter. Evenly top the muffins with a generous spoonful of crumb topping.
Place muffin pan in the oven and bake for 5 minutes at 425°F. Without opening the oven door, reduce temperature to 350°F and continue baking for another 14 to 15 minutes (for a total baking time of 19 to 20 minutes) or until muffins are golden, tops spring back to the touch, and a toothpick inserted in the center comes out clean. Muffins are done baking when the internal temperature reaches 200°F.
Let muffins cool in pan on a wire rack for 10 minutes before removing from pan to cool.