1/2 pound (2 sticks) unsalted butter at room temperature
2 1/2 cups sugar, divided
4 extra-large eggs, at room temperature
1/3 cup grated orange zest (6 oranges)
3 cups all-purpose flour
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1 teaspoon kosher salt
3/4 cup freshly squeezed orange juice, divided
3/4 cup buttermilk at room temperature
1 teaspoon pure vanilla extract
Heat the oven to 350 degrees F. Grease and flour 2 (8 1/2 by 4 1/2 by 2 1/2-inch loaf pans. Line the bottoms with parchment paper.
Cream the butter and 2 cups of the granulated sugar in the bowl of an electric mixer fitted with the paddle attachment for about 5 minutes, or until light and fluffy. With the mixer on medium speed, beat in the eggs, 1 at a time, and the orange zest.
In a large bowl, sift together the flour, baking powder, baking soda, and salt. In another bowl, combine 1/4 cup of the orange juice, the buttermilk, and vanilla. Add the flour and buttermilk mixtures alternately to the batter, beginning and ending with the flour. Divide the batter evenly between the pans, smooth the tops, and bake for 45 minutes to 1 hour, until a cake tester comes out clean.
While the cakes bake, cook the remaining 1/2 cup of granulated sugar with remaining 1/2 cup orange juice in a small saucepan over low heat until the sugar dissolves. When the cakes are done, let them cool for 10 minutes. Take them out of the pans and place them on a baking rack set over a tray. Spoon the orange syrup over the cakes and allow the cakes to cool completely.
Source: Ina Garten