Preheat oven to 300°f and lightly grease an 8" square baking pan*. Combine milk and vinegar in a measuring cup and set aside.
Put all dry cake ingredients (flour, sugar, baking soda, cinnamon, salt) in a mixing bowl and whisk to combine
Stir milk/vinegar, avocado oil and egg into dry ingredients
Heat apple cider until hot but not boiling and stir in vanilla. Wait a few seconds to allow vanilla to bloom and then pour into batter. Whisk until smooth. Batter will be thin, this is ok!
Bake cake in center of preheated oven for 45 minutes. Cake is done when center springs back when lightly pressed.
Make the salted caramel while the cake bakes (recipe follows)
Remove cake from the oven, let it cool 10 minutes, then poke holes all over the top with a wooden skewer.
Pour 3/4 cup of the warm caramel evenly over the warm cake and spread it across the top, letting it seep down into the holes. Let cake cool completely. Then cover and chill 1 hour.
Once cake is chilled, make frosting (recipe follows), spread over the top and chill overnight to let the cake absorb the caramel and develop it's dense texture. (it's good the day it's made but it gets better as it sits so try to be patient and give it a full day before you dig in- you'll be happy you waited, I promise!)