In a saucepan, melt the butter over medium heat until it turns golden brown and has a nutty aroma. Remove from heat and let cool slightly.

In a mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt.

In another bowl, combine the browned butter, granulated sugar, brown sugar, eggs, and vanilla extract. Mix until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Fold in the grated carrots and walnuts if using.

Pour the batter into a greased baking pan and spread evenly. Bake at 350?F (175?C) for 25-30 minutes or until a toothpick comes out clean.

Allow the bars to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Drizzle with cream cheese frosting if desired before serving.