Combine the flour, cocoa, granulated sugar, baking powder, baking soda, and salt together in a large bowl.
Add the eggs, sour cream, oil, 1 tsp vanilla, vinegar, and 1 cup hot water and mix until the batter is smooth.
Beat 1 cup butter and jam until fluffy. Add 1 1/2 cups confectioners sugar and beat until smooth.
Spread betweed the two cakes.
Beat 1 cup butter and bitersweet chocolate (melted and slightly cooled) until fluffy. beat in fluff, 3/4 tsp vanilla, and 6 tbsp confectioners sugar. Spread over the two cakes.