Preheat the oven to 340°F.
Line three 6-inch round cake tins.
Mix the milk and vinegar, then set aside to thicken.
Add the remaining wet ingredients to the thickened milk mixture.
Sift in the dry ingredients and mix until combined.
Pour the batter into the tins.
Bake for 25-35 minutes, or until the cake springs back when touched and a knife inserted into the center comes out clean.
Let the cakes cool before leveling them off.
Using a hand mixer or stand mixer with a paddle attachment, cream the butter on medium-high until creamy and light in color. With the mixer on low, add the powdered sugar bit by bit, scraping down the sides of the bowl after each addition. Add the cocoa powder and milk, and mix until well combined.
Mix together the ingredients for the caramel.
Once everything is cool, assemble the cake with layers of chocolate buttercream, crushed pretzels, and caramel.