Preheat oven to 400 degrees F (200 degrees C).
Whisk All-Purpose Flour (2 cup), Salt (1/2 teaspoon) and Baking Powder (3 teaspoon).
Melt two thirds of the Unsalted Butter (4 tablespoon) and combine with the Granulated Sugar (1/2 cup), 5 tsp of zest from the Sunkist Orange (3/4), Egg (2), Vanilla Extract (1 teaspoon), and Orange Juice (1/2 cup).
Make topping by combining the remaining 2 tsp of zest from the Sunkist Orange (1/4) and Unsalted Butter (2 tablespoon), Ground Cinnamon (1 teaspoon) and Brown Sugar (1/2 cup).
Add the liquid ingredients to the flour mixture and mix until combined. Pour into a greased 8-inch pan. Sprinkle with topping. Bake for 30 minutes.
Let cool slightly on a wire rack before eating.