In a small bowl, mix together the milk and vinegar. Set aside to "sour".
Preheat the oven to 350°F. Butter and flour three 8-inch cake pans. Set aside.
In a large bowl, cream together the butter, shortening and sugar until the mixture gets white, around 4 minutes.
Add in the eggs one at a time, mixing after every addition.
In a medium bowl, sift together the flour, baking powder and salt. Mix the vanilla into the soured milk.
Alternate between adding the dry ingredients and the milk mixture into the wet ingredients. Mix together until the ingredients are fully combined.
Pour the batter evenly into the prepared pans and bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the centre comes out clean. Allow the cakes to cool slightly in the pan for around 5 minutes, then transfer to wire racks to cool completely.
In a large bowl, cream together the buttercream ingredients. Frost the cake as desired. Refrigerate. Top with fruit, flowers and/or ganache drip.