Butter an 8 x 8-inch square baking dish and line with parchment paper.

Preheat the oven to 325°F with a rack positioned in the center.

In a food processor, pulse the sugar, flour, and salt. Add the butter and zest and pulse just until a dough forms. Press the dough evenly across the bottom of the baking pan. Bake the crust for 20-25 minutes, or until medium golden. Allow to cool.

In a saucepan, whisk the egg yolks with ¾ cup / 150 g of the sugar and the orange and lemon juices, along with the salt. In a small bowl, whisk the cornstarch with the water. Add the cornstarch mixture to the saucepan and cook over medium, whisking, until the sugar dissolves - about 3 - 5 minutes. Dial up the heat and simmer, stirring constantly, for another minute or two. The mixture should be thick and glossy, the consistency of a thick honey. Strain the citrus mixture into a small, heatproof bowl, add the orange zest and allow to cool.

In a large bowl, either using a stand mixer (or by hand), mix the cream cheese with the remaining 1 cup / 200g of sugar until smooth. Mix in the flour, and after that add the eggs one at a time. Stir well and scrape down the sides of the bowl regularly while mixing. Mix in the sour cream. Scrape the vanilla bean paste from the inside of the bean if using, add it and the vanilla extract. Mix until the batter is smooth and uniform.

Pour the cream cheese batter over the prepared crust, and smooth the surface with a spatula until level.

Dollop the citrus mixture across the cheesecake batter. Once it is all on, use a chopstick or the handle-side of a spoon to carefully swirl the mixture into the batter. You want to create a pretty swirl pattern. Don't overdo it, and avoid going too deep and accidentally disrupting the crust. Less (swirling) is more here.

Bake the cheesecake for about 50 minutes or so. The cheesecake bars are finished when they are beautifully golden around the edges, puffed up, and just set in the center.

Remove from oven and place the baking dish on a cooling rack. Run the tip of a knife all around the edge to separate the cheesecake from the pan, and to lessen the likelihood of cracks. Cool on the rack, then refrigerate until completely chilled. Cut into bars to serve.