Place the flour, starter, sugar, pumpkin, butter and milk in a large mixing bowl and stir until combined. Cover tightly and place in the refrigerator for 24 hours. 

The following day, remove the dough mixture and let sit out on the counter for about 4 hours so it reaches room temperature. 

Preheat the oven to 350ºF and adjust the rack to the middle position. Line a muffin tin with unbleached parchment liners. 

Transfer the flour mixture to the bowl of a standing mixer. You can also use hand-held beaters. 

In a medium-size bowl, combine the zucchini, vanilla, eggs and yogurt. Whisk to combine. Stir in the cinnamon, nutmeg, baking soda and salt and then immediately, add the zucchini mixture into the flour mixture and mix on low using the beater. Mix until just combined and do not over mix (this will make the dough tough). 

Spoon equal portions of dough into each muffin cup, filling the cups about 3/4 full. Bake for 25-28 minutes, until just turning golden brown. Remove from oven and cool. Repeat with remaining batter. 

Leftovers can be frozen and then reheated in the oven, or store in the fridge for 3-4 days and reheat in the oven before serving.