Put your chocolates in a big bowl. Put the sugar and 1/2 cup water in a saucepan and place over medium heat. Stir until the sugar dissolves and bring up to a boil. As soon that happens, stop stirring and let it boil until it turns an amber color, 8 to 10 minutes or so. (If you think that it needs another stir, brush down the sides of the pan with a wet pastry brush, and stir the entire mixture, picking up the pot itself by the handle, being careful not to touch the caramel or else it could crystallize.) Once the mixture reaches that dark amber color, turn the heat down. Stand back from the pot and add the cream very slowly, stirring with a heatproof spatula. (If you are nervous about the steam, wear a kitchen glove to protect your hand.) Continue to stir until all is incorporated. Once the mixture is smooth, stir in the fine salt. Take off the heat and pour the caramel over the bowl of chocolate. Whisk to combine, making sure all the chocolate melts. Pop in the fridge for about 1 hour until completely chilled.
Take the frosting out of the fridge and beat it with a hand mixer just until it becomes spreadable. Add the butter in pieces and mix until it's all combined. Pop back into the refrigerator for another hour.
Place 1 cake layer on a cake platter, flat-side up. (You want the cake flat-side up so when you get into it you'll have nice and even layers.) Evenly spread 2 cups of the frosting over the top. Sprinkle with sea salt flakes, then put the other cake layer on top, flat-side up. Spread the rest of the frosting over the top and on the sides of the cake. Sprinkle the top with more salt flakes.
Put a rack in the middle of the oven and preheat the oven to 350 degrees F. Butter 2 (9 by 2-inch) nonstick round cake pans. Line the bottom of each with a round of parchment paper, and then butter the paper.
Sift the flour, cocoa, baking soda and salt together onto parchment paper. With an electric mixer on medium-high speed, cream the butter and sugars until light and fluffy, 4 or 5 minutes. Add your eggs, 1 at a time, mixing until completely incorporated before adding each additional egg. Mix in the chocolate and vanilla, making sure to combine really well. Turn the mixer to low and slowly add the dry ingredients, alternating with the buttermilk, in 3 batches, starting and ending with the flour mixture. Make sure to let it mix thoroughly.
Pour half the batter in each pan and spread it evenly with a spatula or butter knife. Bake until a toothpick comes out clean, 25 minutes. Remove the pans to racks and allow the cakes to cool for 10 minutes or so. Loosen the edges of the cake from the pan with a knife, then turn them onto the racks. Remove the paper and let the cakes completely cool.