Preheat the oven to 325°F (160°C) convection (with the fan) or 350°F (175°C) conventional (no fan). Line 12 cupcake pans with cupcake liners.

In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.

In a separate large mixing bowl, whisk the eggs until smooth. Slowly whisk in the oil to emulsify. The mixture should be pale and voluminous.

Whisk in the granulated sugar and light brown sugar until smooth.

Fold in the flour mixture. When the dry ingredients are almost completely incorporated, then stir in the shredded carrots and chopped walnuts (if using). Fold in to distribute evenly.

Divide the batter evenly between the prepared muffin cups and bake in the preheated oven for 10-18 minutes or until a toothpick inserted into the center comes out with only a few clinging crumbs. Avoid over-baking to ensure moist cupcakes.

Allow cupcakes to cool in pans for 2 minutes, then turn them out onto a wire rack to cool completely before frosting.

While the cupcakes cool, prepare the buttercream. In a large mixing bowl with an electric mixer, cream together the softened cream cheese, butter, and powdered sugar until smooth and well combined.

Add the lemon juice and vanilla extract. Mix well until the frosting is light and fluffy.

Place a large round piping tip (such as Ateco 808 or 809) in a piping bag with the tip cut off. Fill with the chilled buttercream (if it becomes too stiff, refresh it briefly in the stand mixer) and pipe a generous swirl onto the top of each cooled cupcake.