Preheat oven to 350 degrees with the baking rack in the middle. Line 2 large 12" x 18" or 15" x 21" baking sheet pans lined with parchment or silicone mats & set aside (alternatively butter and flour the sheet pans).
*For round cake - butter and flour three 8-inch round cake pans*
Mixture 1: In a bowl of a stand mixer whip together 4 eggs, 1 cup sugar, and ½ tsp salt for about 10 minutes. Eggs should be whipped until the mixture flows off the beater in a ribbon that takes 2-3 seconds to disappear.
Mixture 2: When the eggs are almost done whipping, in a medium bowl whisk together 1 cup sour cream, 1 can dulce de leche and 1 tsp of baking soda until no lumps of dulce de leche are seen. The mixture will increase in volume from the baking soda, so make sure to use big enough bowl.
Combine: Fold the two mixtures together, being careful not to overmix and deflate the batter.
Add the dry: Sift in 2 cups of flour and 2 teaspoons of baking powder, then fold the batter until fully incorporated. Do not overmix or the cake will be tough.
Pour & Bake: Divide the batter between the 2 prepared baking sheet pans (or 3 rounds ones for a round cake) into an even layer. Bake for about 9 minutes (about 12 min for round cakes) or until the top is lightly golden and springs back when touched. Remove from the oven and completely cool.
Roast: Preheat oven to 325 with the baking rack in the middle.
Place the hazelnuts onto a rimmed baking sheet and bake for about 15 minutes, tossing the nuts about every 4 minutes and tasting to see if they're roasted to your liking. Remove from the oven and allow to cool completely.
Skin the hazelnuts: Add the nuts to a ziplock bag and keep shaking and rubbing the hazelnuts between the palms of your hands until the skin is removed.
Next, shake the bag so that hazelnut skins sink to the bottom of the bag, and remove the nuts to a separate bowl, discarding the skins.
Leave 20 or so hazelnuts whole, chop the rest of the nuts with a sharp knife to a medium size, or pulse a few times in the food processor.
Combine the evaporated milk and the dulce de leche with a whisk or an immersion blender until smooth. Pour into a squirt bottle if available.
If using heavy whipping cream instead of whipped topping, whip the cream with a mixer until medium peaks and refrigerate.
Whip 8 oz cubed room temperature butter and 8 oz cubed cream cheese for about 4-5 minutes or until the mixture is very fluffy. Scrape down the bowl several times throughout. Add the dulce de leche in 2 additions and whip after each.
Fold in the refrigerated whipped cream (or whipped topping) into the caramel cream in two additions.
Flip each cake upside down and peel off the parchment paper or the silicone mat from the back. Cut each cake in half width-wise so that you have four cake layers total.
Place a dab of frosting onto the serving plate. Now top with the fist cake layer and press to adhere. Using a squeeze bottle or a spoon, pour 1/4th of the Caramel Soaking Syrup evenly all over the cake layer.
Reserve about 2 cups of cream and set aside (1 1/2 cups for a round cake). Layer the cake by soaking each layer with the soaking syrup, then spreading some caramel cream and sprinkling some chopped hazelnuts.
Add the final layer, then use 1 1/2 cups of caramel cream (1 1/4 cups for round) from that which was set aside to cover the top and sides of the cake in icing.
Pipe the leftover 1/2 cup of cream (1/4 cup for round) on cake top and decorate with whole roasted hazelnuts (use chopped hazelnuts for round version).
Refrigerate the cake for about 2-3 hours before serving. Serve cold or room temperature.