1. Preheat oven to 350ºF. Line a 12-cup muffin tin with paper liners. Whisk eggs and milk in a measuring cup. In a medium bowl, whisk flour, baking powder, cinnamon and salt.
2. In a large bowl, using an electric mixer on medium-high speed, beat butter and brown sugar until fluffy, about 3 minutes. Scrape down sides of bowl. Reduce mixer speed to medium-low, beat in half of egg mixture, then half of flour mixture, beating well after each addition and scraping sides of bowl when necessary. Add remaining egg mixture and remaining flour mixture. Beat on medium speed for 1 minute. Fold in carrots and walnuts, if desired.
3. Divide batter among muffin cups (batter will almost fill cups). Bake until cupcakes are golden and a toothpick inserted into center of 1 comes out clean, 22 to 25 minutes. Let cupcakes cool in pan on a wire rack for 5 minutes, then transfer cupcakes to wire rack to cool completely.
4. Using a rubber spatula, stir cream cheese until smooth in a medium bowl. Stir in honey. Spread or pipe over tops of cupcakes and serve.