Begin by preheating the oven to 165 ºC. Grease 3 round cake pans, 9 inches in diameter. Combine the buttermilk and soda and put it aside. Put the butter, shortening, oil, and sugar in a large bowl, and whisk them. Add the egg yolks, one at a time, beating the mixture well. Then, mix the buttermilk mix and flour, alternatively, into the creamed mixture.