Pour your milk into a glass measuring cup (or medium bowl, but I'm all about fewer dishes) and add the vinegar to make sour milk. Set aside for 5 minutes while you prepare the rest of the ingredients.
In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
Add the egg & butter to the sour milk and whisk to combine. Pour the milk mixture into the flour mixture, whisking constantly until evenly combined. Let the batter rest for 3 mins. This is when the magic happens.
While the batter rests, heat a large skillet over medium to medium-high heat. Pour 1/4 cup of the batter into the pan, repeat until skillet has 3 to 4 pancakes in it.
Cook until a few bubbles form and the edges look cooked - 3 to 4 minutes. (I usually don't see as many bubbles on the surface of the pancakes as you would with a store-bought mix.) Flip the pancake and cook until golden brown, 3 to 4 minutes.
Top with butter and syrup if desired, and serve warm.
You will not taste the vinegar in these pancakes. Don't skip it! The sour milk is along the lines of a buttermilk (or milk + lemon juice buttermilk substitute), but different. You want the vinegar to react with the baking soda to help make the pancakes fluffy.