Preheat the oven to 325°F. Butter two 9-inch round cake pans, line with parchment paper, and butter and flour the paper, shaking out the excess.
In a bowl, sift or whisk the flour, baking soda, and salt together.
In the bowl of a stand mixer (or a large mixing bowl and hand-held mixer), beat the butter until it is well combined. Add the sugar a little at a time and beat the mixture until it is light and fluffy. Beat in the eggs, orange zest, and vanilla.
Beat one-third of the dry ingredients into the butter/sugar mixture, then mix half of the buttermilk until combined well. Mix half of the remaining dry ingredients till combined, then add the remaining buttermilk and beat until combined. Finally, beat in the remaining dry ingredients until mixture is smooth.
Evenly divide the batter between the pans, smooth the surface, rap each pan on the counter to expel any air pockets or bubbles, then transfer to the oven. Bake for 45 minutes or until a cake tester or toothpick inserted in the center comes out clean. Transfer to racks and cool in the pans for 20 minutes.