Preheat oven to 350°F. Place the oven rack in the middle of the oven.
Spray a 9x13 inch cake pan with non-stick baking spray and set aside.
Prepare the carrot cake by following the instructions on the package. If using a Duncan Hines Carrot Cake mix, you will have to soak the carrots and raisins in hot water, and whisk all the ingredients listed into the cake mix.
Pour batter into the prepared pan and bake for 30-32 minutes, or until a toothpick comes out with just a few moist crumbs. I baked mine for exactly 30 minutes.
Remove cake from the oven. Using the end of a spatula or a large straw, even a pen will work if you don't have anything on hand; poke holes all over the cake.
In a medium, microwave-safe bowl, combine white chocolate chips and sweetened condensed milk. Heat in the microwave until hot for about 1 minute. Stir and heat 30 seconds or more if needed. Whisk until fully combined, smooth and shiny.
Pour the chocolate mixture all over the cake and using a spatula spread it evenly into the holes.
You can sprinkle a few ground walnuts onto the cake now, so they get into the poked holes.
Using an electric mixer or stand mixer fitted with the whisk attachment, cream together cream cheese, mascarpone and butter on medium speed until smooth with no lumps, about 1 minute. Stop once to scrape the sides and bottom of the bowl with a spatula.
Add in vanilla extract and salt, and continue mixing until combined.
With the mixer speed on medium-low, gradually add in the powdered sugar. Increase mixer speed to medium-high and beat until combined. Stop once to scrape the sides and bottom of the bowl with a spatula.
If frosting is too thick, you can add in one or two teaspoons of milk. If frosting is too thin, you can add in more powdered sugar, 1/3 cup at a time, but make sure you first taste it, so you don't make it too sweet. Also, consider that the frosting may look thin, but it will harden/settle once chilled.
Sprinkle ground walnuts on top of the cake.
Refrigerate cake for at least 6-8 hours before serving.