1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray.

2. In a large mixing bowl, whisk together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt until well combined and no lumps remain.

3. In a separate medium bowl, whisk together the melted unsalted butter, granulated sugar, and light brown sugar until well combined. Beat in the eggs one at a time until fully incorporated. Stir in the sourdough discard and vanilla extract until the mixture is smooth.

4. Pour the wet ingredient mixture into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined, being careful not to overmix. A few streaks of flour are acceptable.

5. Gently fold in the grated carrots, and if using, the chopped walnuts or pecans, and shredded coconut. Mix just enough to distribute them evenly throughout the batter.

6. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full.

7. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back lightly when touched.

8. Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.