Tip: If you opt to use purchased baby food rather than freshly pureed carrots, purchase organic baby food as it will yield better flavor and color.
Preheat oven to 350 F.
Lightly coat the Bundt pan and tunnel insert with cooking spray and dust with flour; set aside.
In a small bowl, sift together the flour, sugar, cinnamon, baking soda and salt; set aside.
In a large bowl, using an electric mixer set on medium speed, beat together the eggs, carrot puree, and vegetable oil. Add the dry ingredients and, with the mixer set on medium-high speed, beat until smooth, stopping to scrape down the sides of the bowl as necessary. Stir in the carrots and raisins.
Spoon the batter into the prepared Bundt pan and spread it around evenly with a rubber spatula. Place the tunnel insert on top of the pan. Bake for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean. Let the cake rest, set on a rack, for 5 minutes.
Slowly remove the tunnel insert. (Note: If the cake has risen over the pan rim, trim off the excess before removing the insert.) Let cool for 10 minutes and then place a cooling rack on top of the cake, invert the cake, and remove the pan. Cool completely.
To make the filling: In a medium bowl, using an electric mixture set on medium speed, cream the butter and cream cheese until light and fluffy. Beat in the vanilla. Add the powdered sugar, reduce the mixer speed to low, and mix until light and fluffy, about 5 minutes.
To fill the cake, invert the cooled cake to reveal the tunnel. Fill the tunnel with the cream cheese filling, spreading it evenly to the outer edges of the cake with a rubber spatula. (Note: It will look like the entire bottom of the cake is frosted.) Sprinkle chopped walnuts over the filling. Invert the cake on to a serving platter.
Chill at least 1 hour before serving. Before serving, dust lightly with powdered sugar.