In a large bowl, sift together the flour, baking powder, salt, and sugar. Whisk to combine evenly.

In a separate medium bowl, whisk together the milk, egg yolk, melted butter, and vanilla extract until well blended.

Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are perfectly fine. Set aside.

In a clean, dry bowl (preferably glass or metal), beat the egg white with the cream of tartar (if using) until stiff peaks form. This means when you lift the whisk, the egg white stands up straight without drooping.

Gently fold about one-third of the whipped egg whites into the pancake batter to lighten it. Then, carefully fold in the remaining egg whites until just incorporated. Be gentle to maintain as much airiness as possible.

Heat a lightly oiled griddle or non-stick frying pan over medium-high heat. You can test if it's ready by flicking a drop of water onto the surface; it should evaporate quickly.

Pour or scoop the batter onto the griddle, using approximately ¼ cup for each pancake. Cook for about 2-3 minutes per side, or until golden brown and cooked through. Bubbles forming on the surface are a good indicator that it's time to flip.

Serve immediately with your favorite toppings.