Salted Caramel Chocolate Sheet Cake

For the cake: Preheat oven to 350 F. Spray a 9 x 13in pan with cooking spray. Sift together flour,cocoa powder, baking soda, and salt in a bowl. With an electric mixer on medium-high speed, cream the butter and sugars until light and fluffy, 4 or 5 minutes. Add eggs, one at a time, mixing until fully incorporated before adding each additional egg. Mix in chocolate and vanilla extract, making sure to combine really well. Turn mixer to low. Slowly add the dry ingredients, alternating with the buttermilk, in three batches, starting and ending with the flour mixture. Make sure to mix thoroughly. Pour batter into prepared pan and spread evenly. Bake until a toothpick comes out clean, 25 minutes. Let cake cool completely. Frost with prepared frosting. For the frosting: Pour chocolate baking melts into a large bowl. Pour sugar and 1/ 2 cup water into a saucepan and place over medium heat. Stir until sugar dissolves and bring up to a boil. As soon as this happens, stop stirring and let it boil until it turns an amber color, around 8 to 10 minutes. (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 saucepan and add the heavy cream very slowly, stirring with a heatproof spatula. Continue stirring until all cream is incorporated. Once the mixture is smooth, stir in the sea salt. Take off the heat and pour the caramel over the bowl of chocolate. Whisk to combine, making sure all the chocolate melts. Place in the fridge for 1 hour, or until completely chilled. Take frosting out of the fridge and add butter in pieces. Beat it with a mixer just until it becomes spreadable. Place in the fridge for another hour.