Preheat oven to 350°F (175°C) and grease two 8-inch round cake pans.
In a bowl, combine flour, baking powder, salt, and cinnamon.
Beat butter and sugar until fluffy, then add eggs and vanilla.
Alternate adding dry ingredients and milk, beating until smooth.
Pour batter into pans and bake for 30-35 minutes, or until a toothpick comes out clean.
While cakes cool, make the frosting: melt butter and brown sugar in a saucepan.
Stir in cream, vanilla, and sea salt. Simmer for 3-4 minutes, then let cool.
Gradually add powdered sugar to the cooled caramel, beating until smooth.
Frost the cooled cakes with salted caramel frosting.
Drizzle with caramel sauce and sprinkle a little extra sea salt on top.