STEP - 1
Mix flour, baking powder, 1 teaspoon cinnamon, sugar, and salt in a medium bowl. Melt 2 tablespoons of the butter. In a large bowl, whisk to combine egg yolk, vanilla, milk, and melted butter. Gently fold dry ingredients into wet ingredients until just combined, then fold in 1½ cups of chopped apple.
STEP - 2
In another medium bowl, using a hand mixer, beat egg white until stiff peaks form. Fold egg white into batter gently with a rubber spatula until just combined.
STEP - 3
Make apple topping: in a small saucepan over medium heat, melt remaining 2 tablespoons butter. Add remaining apples, brown sugar, remaining 1/2 teaspoon cinnamon, and a 1/4 cup water. Cook, stirring occasionally, until apples have softened and mixture is jammy, 10 to 15 minutes.
STEP - 4
In a large frying pan over medium heat, melt 1 tablespoon butter. Pour about 1/4 cup pancake batter into pan. When little bubbles appear, about 1 to 2 minutes, flip and continue cooking until both sides are lightly golden. Repeat with remaining batter, adding more butter to pan as needed. Serve pancakes topped with apple mixture.