In a small bowl, mix the milk and apple cider vinegar and let it sit for about 5 minutes to create a buttermilk substitute.
Combine the dry ingredients
In a large mixing bowl, whisk together millet flour, almond flour, sugar, baking powder, baking soda, and salt.
Mix the batter
Add the egg, vanilla extract, and melted butter to the buttermilk mixture, then pour it into the dry ingredients. Stir until just combined; avoid overmixing.
Heat the skillet
Place a non-stick skillet or griddle over medium heat and grease lightly with butter or oil.
Cook the pancakes
Pour 1/4 cup of batter onto the skillet for each pancake. Cook until bubbles form on the surface and the edges look set, about 2 minutes. Flip and cook for another 1-2 minutes until golden brown.
Serve warm
Stack the pancakes and serve with your favorite toppings such as fresh fruit, maple syrup, yogurt, or nut butter.