Make the cake: Heat oven to 350 degrees F. 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 sweetened condensed milk in a 2-quart 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 3/4 cup pecans over sugar mixture and set aside to cool. Combine the flour, cocoa, granulated sugar, baking powder, baking soda, and salt together in a large bowl. Add the eggs, sour cream, oil, vanilla, and vinegar, and 1 cup hot water and mix with a wooden spoon until the batter is smooth. Pour the batter into the cake pans — over sugar mixture — and bake until a wooden skewer 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.
Assemble the cake: Spread the fudge topping over the sugar mixture side of one cake layer. Place the second layer over the first and drizzle with the melted chocolate and the remaining 1/4 cup pecans. Frost the cake sides with your favorite chocolate icing.