Preheat oven to 350F. Prepare 2 or 3 8-inch round cake pans, as well as 8-inch parchment paper rounds. Set aside.
Prepare the chocolate cake dough. In a large bowl or measuring cup, combine the sweetened condensed milk, milk and vanilla extract. Heat in the microwave for 1 1/2 to 2 minutes until very hot, or heat the mixture over a water bath. Meanwhile, measure all the dry ingredients into one bowl: flour, cocoa powder, baking powder and salt.
Gradually begin adding the dry ingredients into the warm milk mixture. Whisk the flour in at first, then as the batter comes together into dough, switch to a spatula. You should have a very soft and slightly sticky dough. Turn dough out onto a floured work surface. With well-floured hands, knead the dough gently until it's uniform and smooth.
Using a sharp knife, divide the dough evenly into 8 pieces. Work with one piece of dough at a time. Place the dough onto a round of parchment paper. Then using a well-floured rolling pin, roll the dough out into a circle. Use your hands to press it to the edges evenly. Transfer the round into a cake pan. Use a fork to pierce the dough all over to prevent air bubbles while baking.
Bake 2 to 3 cake rounds at a time for about 10 minutes. Remove the cake rounds from the oven and cool on a wire rack. Reuse the parchment paper for the remaining dough.
Prepare the frosting. Place the softened butter and cream cheese into a mixer bowl and whisk on medium-high speed for 2 to 3 minutes until the mixture is smooth and creamy. Add in the sweetened condensed milk and mix again for 1 minute. Next, add in the heavy cream, vanilla and salt and whisk on high speed for about 1 minute, just until stiff peaks form. Do not overmix as this will cause the frosting to separate. Watch my 'Whipped Cream Frosting' video recipe for more details.
Assemble the cake. Spread a generous amount of frosting between each cake layer. Use remaining frosting to frost the top and sides of cake, taking extra care to cover all of the cake layers. Smooth down top and sides with a flat spatula. Place cake into refrigerator for at least 24 hours.
Garnish the cake. Heat the heavy cream until it's very hot but not boiling! Add the chocolate melts and stir until smooth ganache forms. Allow the ganache to cool slightly before transferring it into a large ziplock bag. Cut off the tip of the bag with kitchen scissors, then drizzle the chocolate down the sides of the cake and smooth over the top. Garnish the top of the cake with chopped toasted hazelnuts and fruit, if desired.
Set cake into refrigerator to allow the ganache to set. This cake is best after setting for 2 days in the refrigerator. Remove the cake from the fridge about 1 hour prior to serving to allow the cake to thaw.