1. Preheat oven to 350°.
2. To prepare cake, combine first 5 ingredients in a large bowl; beat with a mixer at low speed for 1 minute or until well combined. Add cake mix to buttermilk mixture; beat at medium speed 1 minute or until well blended. Stir in carrot and apple until just combined. Divide batter evenly among 3 (8-inch) round cake pans coated with baking spray. Bake at 350° for 20 minutes or until a wooden pick inserted in the center comes out clean. Cool cakes in pans on a wire rack 10 minutes. Remove cakes from pans; cool completely on wire rack.
3. To prepare glaze, melt butter in a small saucepan over medium-low heat; cook 6 minutes or until lightly browned, stirring frequently. Cool slightly. Combine browned butter, 1 1/2 cups powdered sugar, cream cheese, milk, vanilla, and salt in a medium bowl; beat with a mixer at low speed 2 minutes or until smooth.
4. To assemble cake, place 1 cake layer on a plate; spread 1/4 cup glaze over cake layer. Top with second layer and spread with 1/4 cup glaze. Top with third layer. Stir the remaining 1/2 cup powdered sugar into remaining glaze. Spread remaining glaze over top of cake. Sprinkle with chopped pecans.