Add the almond milk and apple cider vinegar into a small bowl. Give it a few stirs and let it sit for 5 minutes, until the almond milk curdles.
Add the soaked millet, buttermilk, zucchini, almond flour, chia seeds, flaxseed meal, butter, and coconut sugar into a food processor. Process until the zucchini is completely pureed and all the ingredients are well-combined. Right before you’re ready to bake the cornbread, stir in the baking soda.
Grease an 8 x 8 inch/20 x 20 cm square cast iron pan or a 9"/22.5 cm round cast iron pan. Pour the batter in, and bake the cornbread at 350°F/175°C until a toothpick inserted in the middle comes out with a few moist crumbs, but no wet batter, about 35 minutes.
Let the cornbread cool in the pan for at least 20 minutes before slicing.
Leftover cornbread keeps well in an airtight container for 3 days. For longer term storage, freeze in an airtight container for up to 3 months.