Preheat the oven to 350°F. Spray 3, 8x4-inch loaf pans with nonstick cooking spray, followed by lining each with parchment paper. Set aside.
In a large bowl, cream together the butter and sugar with a hand mixer on medium speed until light and fluffy, about 3-4 minutes.
Beat in the eggs one at a time, making sure each one has been incorporated before adding the next one.
If adding vanilla, add it to the batter now.
Alternating between the flour and milk, add each to the butter mixture, beginning and ending with the flour. Beat until just combined, making sure not to overmix.
Pour batter evenly into the prepared loaf pans (about 2½ cups of batter per pan). Bake for 60-65 minutes, or until a toothpick inserted into the center of the cakes comes out mostly clean.
Remove from oven and allow to cool in pans for 10 minutes, then remove from the pans. Strip off the parchment paper and let cool completely on wire racks.