Advance Prep: Allow butter and milk to come to room temperature.
Preheat the oven to 350 degrees F.
In a small bowl, whisk together the milk and apple cider vinegar and allow it to sit for at least 10 minutes.
In a medium bowl, sift together the flour, baking powder, baking soda, and salt. Whisk to combine and set aside.
In a large bowl, cream together the sugar, butter, and vanilla using a stand mixer or an electric hand mixer.
Run the mixer and slowly drizzle the soured milk until combine. Slowly add the flour mix a little bit at a time and again, beat until combined.
Distribute the batter evenly between 3 8" greased and floured cake pans. Bake the cakes on the center rack of the oven for 27-30 minutes, or until the cakes are golden and a toothpick inserted comes out clean.
While the cakes are baking, you can prepare your frosting. Use an electric mixer to whip the butter for a couple of minutes until it's nice and smooth. Add the powdered sugar half a cup at a time and mix until it begins to form a frosting. Add the vanilla extract and whip until the frosting is smooth. If you'd like to color the frosting, add the coloring agent now and mix well.
When the cake is completely cooled, you can gently remove each layer of cake from the pans. If your cakes are a bit rounded at the top and you want them to be perfectly flat, feel free to use a cake leveler (or sharp, serrated knife) to gently slice off the top of each cake. Then stack the layers, one on top of the other, smearing a layer of frosting between each using an icing spatula (or a generic spatula). Beware, if the cake is still warm, the frosting will melt!
Frost the cake exterior using your icing spatula (or kitchen spatula) and decorate as desired.