Preheat the oven to 350 degrees F. Rinse two oranges and pat them dry. Zest the oranges until you have 2 tablespoons of zest, and set it aside. (You will need half of the zest for the crust and the other half for the orange topping.)
For the crust, in a medium mixing bowl, toss the flour, baking powder and salt together. Cut up your stick of butter and add it to the bowl. Use a pastry blender or two knives to cut the butter into the flour mixture until it resembles coarse crumbs. Stir in the powdered sugar and one tablespoon of orange zest.
Press the crust mixture evenly into the bottom of an 8x8 pan. Bake the crust for 7 minutes.
While the crust is baking, prepare your orange topping in the same bowl you used for the crust. Squeeze three tablespoons of orange juice and add it to the bowl. Whisk in the eggs, sugar, one tablespoon of orange zest, flour and baking powder until blended.
When the crust has baked for seven minutes, remove the pan from the oven and pour the chocolate chips on top in an even layer. If the chips aren't melting enough, put the pan back in the oven for two minutes. Remove the pan from the oven and use a spatula to spread the melted chips into an even layer of chocolate.
Pour your orange topping over the chocolate layer. Wearing oven mitts, place your hot pan back in the oven to bake for 35 minutes, or until the top layer is lightly browned and set. (It should not be jiggly.)
Cool the pan on a wire rack. When the dessert has cooled completely (a couple hours is best) sprinkle with powdered sugar and sea salt, if desired. Cut into 12 bars. Store in a plastic container at room temperature for up to 4 days.