Add butter and remaining milk. Mix on low speed until the dry ingredients are moistened. Scrape down the sides. Gradually beat in the egg mixture in 3 batches, beating for 20 seconds after each addition to incorporate the ingredients and strengthen the structure. Scrape down the sides.
Scrape batter into prepared pans, filling about halfway, and smooth with a spatula.
Arrange pans in the oven so air can circulate around them. Do not allow them to touch each other or the oven walls. The 12-inch layers should bake until they just start to shrink from the sides. Do this quickly so oven temperature does not drop. Loosen the sides with a spatula and invert onto greased wire racks.