Preheat the oven to 350˚F. Line two muffin tins with paper liners (makes 14).
In a small bowl, mix together 3/4 cup milk and the vinegar. Set aside to "sour".
In a medium bowl, mix together the flour and the baking powder.
In a separate bowl, cream together 1/2 cup butter and sugar until light and fluffy. Add in the eggs one at a time, mixing well after each addition. Mix in the vanilla extract.
Mix in the dry ingredients and the sour milk, alternating between the two until fully incorporated.
Divide the mixture into the prepared muffin pans. Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted comes out clean. Allow to cool on wire racks.
Meanwhile, cream together all of the buttercream ingredients until smooth. Pipe onto the cooled cupcakes.