To make the buttermilk: Combine milk and vinegar in a bowl and set aside for 5 minutes to sour the milk.

Combine flour, sugar, baking powder, baking soda, and salt in a large mixing bowl.

Whisk egg, butter, and vanilla extract into buttermilk mixture until well combined; add flour mixture and whisk until just combined. Fold in chocolate chips. The pancake batter will be thick. Allow batter to rest for 5 minutes.

Heat a large skillet over medium-low heat, and coat with cooking spray.

Pour about ¼ cup of batter for each pancake onto the skillet. Cook until bubbles appear on the surface, 2 to 4 minutes. Flip pancake with a spatula, and cook until browned on the other side, 2 to 3 more minutes. Repeat with remaining batter. Serve immediately.