1 cup all-purpose flour
2 teaspoons sugar
½ teaspoon salt
½ teaspoon baking powder
¼ teaspoon baking soda
¾ cup buttermilk
¼ cup milk (plus a teaspoon or so if the batter is too thick)
1 large egg, separated
2 tablespoons unsalted butter, melted

Whisk dry ingredients in medium bowl.
Pour buttermilk and milk into 2-cup Pyrex measuring cup.
Whisk egg white into milk mixture.
Mix yolk with melted butter, then stir into milk mixture.
Dump wet ingredients into dry ingredients all at once; whisk until just mixed.