Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners.

In a medium saucepan, melt unsalted butter over medium heat until it turns golden brown.

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

In another bowl, combine cooled brown butter, brown sugar, eggs, vanilla extract, and sourdough discard; whisk until smooth.

Gently pour the wet mixture into the dry ingredients and fold until just combined, then fold in carrots and nuts.

Fill each muffin cup about three-quarters full with batter.

Bake for 18-20 minutes, checking for doneness with a toothpick.

Let muffins cool in the tin for 5 minutes before transferring them to a wire rack.

Prepare cream cheese frosting by beating together cream cheese, butter, and powdered sugar until smooth.