Preheat oven to 350 degrees F. Butter and flour three 6 inch round pans. Whisk together the flour, baking powder and salt. Set aside.
Mix vinegar and milk in a small bowl and set aside.
Cream butter and sugar in a stand mixer using a paddle attachment.
Add vanilla and eggs one at a time mixing until incorporated before adding the next. Scrape the bowl down.
Add flour and milk mixtures to the butter in alternating batches.
Bake at 350F for about 30 minutes or until the centers are set and springy.
For the Caramel:
Add the sugar and water into a medium saucepan and stir until sugar is dissolved. Place over medium-high heat and brush the inside of the pot with a wet pastry brush so crystals do not form.
Cook until sugar mixture becomes a golden amber color, then remove from heat and whisk in the cream vigorously. Add butter and bring back to a medium low heat. Continue whisking and cook for an additional 2-3 minutes.
Mix in salt, transfer to a bowl and set aside to cool to room temperature. Caramel can be made in advance and refrigerated.
For the Frosting:
Whisk together egg whites and sugar in a glass bowl. Set over a pot of simmering water making sure the water doesn't touch the bowl.
Whisk over hear until a thermometer reads 165F. Transfer mixture to a stand mixer fitted with a whisk attachment and mix on high for a few minutes then stitch to medium low until room temp.
Switch to a paddle attachment and add butter a tablespoon at a time. Reserve about a cup and a half of the frosting.
Add about 1/3 a cup of caramel to the large batch of frosting and another 1/3 a cup to the small batch. You can add more caramel and salt to taste but it is important for the two batches of frosting to have a visible contrast.
For the Assembly:
Drizzle caramel onto the fist layer then add a layer of buttercream. Place second layer on top and repeat the process until cake is three layers tall.
Cover cake in buttercream and smooth. Use a cake comb to cut grooves into the side then chill the cake.
Pipe the darker caramel buttercream into the grooves and smooth so stripes show up.
Add a layer of caramel onto the top of the cake and smooth. You can add additional drips onto the side if needed.
Sprinkle flaked sea salt onto the top of the cake and serve.
For the Caramel:
Don't stir! After the sugar has dissolved you don't need to stir as it can cause crystals to form and ruin that silky consistency.
DO brush the side of the pot down with a wet pastry brush. This will help prevent those pesky crystals from forming.
Watch the color carefully taking the sugar to a deep amber color gives a wonderful flavor but going too dark will make a bitter caramel and that sugar will go from perfect to burnt in an instant!
Use room temperature butter and cream.
Caramel keeps for a while in the fridge so you can make this part ahead of time or make a big batch and save some for ice cream, coffee drinks and snacking!
When adding salt dip your a spoon in let the caramel cool a bit then taste and add more salt if needed. You can also sprinkle more salt on at the end for a finishing touch!