In a large glass measuring cup (I have these), measure apple cider vinegar. Add milk until you reach 1 1/2 cups of liquid total. Whisk together and set aside.
In a large bowl, combine all dry ingredients and whisk together (you can use the same whisk). Set aside.
When the oven is preheated, melt 2 Tbsp butter OR bacon grease in a 12″ cast iron skillet over medium heat. Meanwhile, add the egg to the milk and whisk together. Add the baking soda to the milk and whisk to incorporate.
Carefully add wet ingredients to dry ingredients and incorporate well using a spatula, taking care not to over-stir. The batter may be lumpy.
When the skillet is hot and the fat is sizzling, add the melted butter to the batter while stirring, and then immediately add the batter to the skillet.
Set the timer for 1 minute and let the batter cook. The edges will get brown and crispy – you want this!
When the timer beeps, place the entire skillet in the oven and bake for 20 minutes. Allow the cornbread to cool slightly before serving. Serve warm with lots of butter!