Pour buttermilk into a large bowl and soak popcorn for 30 minutes. While that is soaking, preheat oven to 350 degrees. Grease two 7-inch cake pans and set aside.
Sift together the cake flour, baking powder, baking soda and salt; set aside. In a small bowl, whisk together the eggs and vanilla.
Strain buttermilk, being careful to remove all popcorn. Reserve 1 cup for the cake. Using a mixer on medium speed, beat the butter until creamy. Over the course of 3 minutes, beat in the sugar. Over 2 minutes, add the egg mixture. Reduce the speed to low and alternate adding the flour and buttermilk in three parts, scraping the bowl.
Divide the batter between the pans and smooth the tops. Bake until light golden brown and a toothpick inserted in the center comes out clean, 25 to 30 minutes. Cool for 10 minutes, then un-mold onto a rack to cool completely before frosting.