Preheat the oven to 350 F. Line an 8x8 square or 9x9 square light colored metal baking pan with parchment paper (if you grease it first, the parchment sticks to the pan better).
Place the sugar (200g or 1 cup) in a large bowl and zest the (2) oranges over it. Rub the zest into the sugar with your fingers, this will release the oils and flavor from the zest, until the sugar feels wet.
Melt the melted butter (226g or 1 cup) then pour it into the bowl and add the vanilla (2 teaspoons) and salt (¾ teaspoon). Whisk the mixture very well, for about 90 seconds.
One by one add the room temperature eggs (3), whisking for at least 30 seconds after each egg is added. When all the eggs are in you should have a thick, shiny, nicely emulsified mixture.
Sift in the cake flour (300g or 2 ¼ cup) and add the baking powder (2 ¼ teaspoons), start to whisk the mixture and once it’s halfway mixed, add the sour cream (120g or ½ cup) and orange juice (110g or scant ½ cup). Whisk until you have a smooth, even batter.
Pour the batter into the prepared pan and smooth it into an even layer. Sprinkle a generous amount of granulated sugar on top (2-3 tablespoons).
Bake the cake until a cake tester comes out clean, about 40 minutes for a 8x8 pan, slightly less for a 9x9 pan.
Let the cake cool briefly in the pan then use the parchment as a sling to lift it out.
Store leftovers in an airtight container, they’ll keep at room temperature for about 3 days but the cake is best eaten within the first 48 hours.