Center a rack in the oven and preheat to 350 (F). Prepare 3 six-inch cake pans by liberally spraying with cooking spray and lining the bottom of each pan with a parchment paper circle. Spray the parchment paper as well, and set aside.
In a small bowl, combine 1 cup granulated sugar and the fresh zest from 1 large orange. Use your fingers to rub the zest into the sugar, incorporating it throughout the bowl to mix the two together. As you rub the zest into the sugar, your sugar should become fragrant as you're helping release oils from the zest. Set aside when the two are well combined.
In a medium bowl, whisk together 1 2/3 cups all-purpose flour, 2 teaspoons baking powder, and 1 teaspoon kosher salt. Set aside.
In the bowl of a freestanding electric mixer fitted with a paddle attachment, combine 3/4 cup plus 2 tablespoons unsalted butter and the orange-infused sugar (from the 2nd step). Beat on medium speed until light colored and fluffy, at least 10 to 12 minutes. Add 10 large egg yolks in 3 additions, beating until completely combined after each, about 2 minutes per addition, and stopping to scrape down the sides of the bowl with a rubber spatula before adding more yolks. Once the yolks have all been added, increase the mixer speed to its highest setting and beat for another 30 seconds to fully combine, before turning the mixer off completely. Use a rubber spatula to scrape down the sides of the bowl. In a liquid measuring cup, whisk together 2/3 cup buttermilk and 1/2 teaspoon pure vanilla extract. Set aside.
Turn the mixer back on to its lowest setting. With the mixer on low speed, add the dry ingredients (from the 3rd step) in 3 additions alternating with the buttermilk mixture (from the the 5th step) in 2 additions, beginning and ending with the dry ingredients. After each addition, beat until just combined, being careful not to overmix. It's best to stop the mixture early with one or two flour streaks left, and finish the rest by hand by scraping down the bowl and folding the mixture one or two more times.
Use a cookie dough scoop to divide the batter evenly between the prepared pans, using an offset spatula to smoothen the tops. Bake in the preheated oven for around 35 to 40 minutes, or until the cakes are lightly brown and a skewer inserted in the center of each cake comes out clean Transfer the cakes to wire racks and let cool for about 20 minutes, before turning out onto the wire racks to cool completely.