Make Cake: Preheat oven to 350º. Butter the bottoms of two 9 inch round cake pans and fit a circle of parchment paper large enough to cover 1 inch up the side of each cake pan. Heat the butter, brown sugar, and condensed milk in a saucepan over medium heat until the butter melts and the sugar is dissolved. Do not boil. Divide the sugar mixture between the prepared pans. Sprinkle ¾ cup pecans over sugar mixture and set aside to cool. Combine flour, cocoa, sugar, baking powder, baking soda, and salt together in a large bowl. Add eggs, sour cream, oil, vanilla, and vinegar, and 1 cup hot water and mix with spoon until the batter is smooth. Pour the batter into the cake pans - over the sugar mixture – and bake until a toothpick inserted into the cake center comes out clean (35 to 40 minutes). Cool cakes in the pans for 10 minutes. Run a knife around the edge of the pans and turn cakes out onto a cooling rack. Remove the paper and cool completely.
Assembly of the cake: Spread fudge topping over the sugar mixture of one cake layer. Place the second layer over the first and drizzle with the melted chocolate morsels and the remaining ¼ cup pecans. Frost the sides with Chocolate Fudge Buttercream Frosting.