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.
Notes
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.