These bars get their flavor from the orange-flecked cream cheese custard on top of a cakey base. Delightfully soft and smooth, they are a great blank canvas for flavoring with any sweet orange citrus — like clementines, mandarins, Cara Caras, tangerines — or lemon. Don’t try to reduce the sugar in the filling — these bites are intentionally rich.
Heat the oven to 325 degrees. Grease a 9-by-13-inch metal baking pan, then line the bottom and two long sides with parchment paper; grease the paper with butter.
Make the cake base: In the bowl of a stand mixer fitted with a paddle (or in a large bowl and using a hand mixer), whisk together the flour, sugar, cornstarch, salt, baking powder and baking soda. Add the butter, egg and oil and beat on low speed until the dough comes together and there are no dry patches of flour remaining. Scrape the dough into the prepared baking pan and reserve the bowl (no need to wash it). Use your fingers or a small silicone spatula to press the dough evenly over the bottom of the pan.
Place the cream cheese in the bowl and beat on medium-low speed until soft, about 30 seconds. Reduce the mixer speed to low and add the powdered sugar, one spoonful at a time. With the mixer still running, add the vanilla, orange zest and eggs and continue mixing until smooth. Scrape the paddle and bowl with the spatula and stir to make sure the custard is smooth.
Scrape the custard over the cake base and spread it to create an even layer. Bake, rotating the pan front to back halfway through cooking, until golden brown at the edges and the center wobbles slightly when you tap the side of the pan, 35 to 40 minutes. Transfer the pan to a rack and let the cake cool completely in the pan.
Use the parchment paper to lift the cake slab out of the pan and onto a cutting board. Cut the cake into squares and garnish each with more fresh-grated orange zest and/or flaky sea salt, if you like.
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