Preheat the oven to 350° F. Butter 2 (8-inch) round cake pans and line with parchment paper. In a large mixing bowl, beat together the butter and sugar until light and fluffy. Add the vanilla, beating until smooth. Beat in the eggs, one at a time, until combined. Add the flour, baking powder, baking soda, and salt. Stream in the buttermilk, beating until just combined. Pour the batter among the 2 cake pans.Bake for 25 to 30 minutes, until the center is just set. Let cool 5-10 minutes. Run a knife around the edge of the cake and invert the cake onto a plate, flip it back over onto a serving plate. Let cool.To make the chocolate frosting. Microwave the chocolate chips and a pinch of salt, stirring every 45 seconds, until melted. Stir in the sour cream and espresso, a dollop at a time, until the mixture is smooth. Swirl the frosting onto the top of the cake. Place the second cake layer on top. Run a knife around the side of the cake to clean up any frosting. Chill while you make the brown sugar frosting.To make the brown sugar frosting. Melt together the butter, brown sugar, and cream in a medium saucepan. Bring to a boil and cook 2 minutes or until the sugar has dissolved. Add a pinch of salt. Remove from the heat and add to the bowl of a stand mixer. Sift in the powdered sugar, then add the vanilla. Beat until well combined. The frosting should be glossy and pourable. If it feels too runny, add powdered sugar to thicken it.Pour the frosting over the cake, spreading around the sides of the cake too. The frosting will set quickly on the cake. If desired top with chocolate curls and sea salt. Slice, eat, and enjoy!!