Heat oven to 350°F. Line muffin pan with paper baking cups or lightly grease.
Whisk together eggs, milk, melted butter and vanilla until frothy. Add cake mix, cinnamon and nutmeg and stir until combined. Stir in carrots and nuts, if using.
Scoop batter into muffin pan, filling cups ⅔ full.
Bake 21-24 minutes or until a toothpick inserted into center comes out clean. Let cupcakes cool completely before frosting.
Using a hand or stand mixer, combine butter and cream cheese and beat until well-combined and smooth.
Add vanilla extract and salt and stir well to combine.
With mixer on low, gradually add powdered sugar until completely combined.
Frost cupcakes as desired. Refrigerate cupcakes once frosted.