Preheat the oven to 325°F. Line the bottom of two 8 inch pans with parchment paper, and butter and flour the sides. Set aside. (See recipe notes for baking instructions for 9 inch pans, cupcake pans or 9 x 13 inch pan).
Whisk the butter, oil, salt and sugar until the mix is creamy and fluffy.
Add the eggs, one at a time, mixing well between each addition. Mix in the milk (and vinegar) mix and vanilla.
In a separate bowl, sieve the flour and baking powder together and whisk to combine.
Fold the flour into the wet ingredients in 2 - 3 additions, and mix until just combined.
Pour the batter into the two prepared pans.
Bake in preheated oven for about 25 - 30 minutes, rotating the pan halfway through baking time. (See notes for baking instructions for 9 inch pans, cupcake pans or 9 x 13 inch pan).
The cake should be springy to the touch (when you gently touch the middle of the cake, it should spring back up). Remove the cake from the oven and let it cool slightly in the pan. While still warm, turn the cake out onto a wire rack and let it cool down completely.
Place one cake layer on a cake serving plate or decorating plate. Place about 1/2 - 3/4 cup of the frosting on top of this cake layer and spread it evenly. You can choose to add a little more of the frosting if you like.
Place the second cake layer on top and press it down gently to make sure the cake layers stick together. Apply a thin coating of the frosting to completely cover the cake. This is the crumb coating. Allow the crumb coating to set in the fridge for a few minutes.
Spread the rest of the frosting over the cake evenly. Use an offset spatula to create swirls for an old fashioned cake look OR use a bench scraper or spatula to smooth out the frosting.
Baking time for 9 inch cake pan - 20 to 25 minutes baking time. Remember to rotate half way through.Baking time for cupcake pan - fill the cupcake liner with the batter about 2/3 of the way. Bake for 15 to 20 minutes. Remember to rotate half way through. Makes 12 cupcakes.Baking time for 9 x 13 inch pan - 28 to 33 minutes. Remember to rotate half way through.