Add the milk, yogurt, eggs, butter, and vanilla to a medium bowl. Whisk to combine into a smooth mixture.
Add the flour, sugar, baking powder, and salt. Whisk gently to combine.
Fold in the apple OR slice the apple ⅕ inch thick.
Warm a nonstick or cast-iron skillet over medium heat. Add a small bit of the butter, melt, and spread it around the pan.
Drop ¼ cupfuls of batter onto the pan and spread out thinly to about ¼-½ inch think. Cook for about 3 minutes or until you see bubbles on the surface of the pancake. Flip and cook for an additional 2-3 minutes. Continue to cook the rest of the batter.
If using the sliced-apple option, dip each one in the batter, flip over, and use a fork to lift out of the bowl and place onto the pan. Cook as directed in the previous step.
Serve warm.
