In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt. In a medium bowl, whisk together buttermilk, applesauce, egg, and butter. Add applesauce mixture to flour mixture and whisk until just moistened (batter should have some lumps; do not overmix).
Lightly coat a large nonstick skillet or griddle with cooking spray and heat over medium-high. In batches, drop batter by 1/4 cupfuls into skillet. Sprinkle each pancake with diced apple. Cook until bubbles appear on top and a few have burst, 2 to 5 minutes. Flip pancakes and cook until golden brown on underside, 2 to 5 minutes. Repeat with more cooking spray and remaining batter, wiping skillet clean between batches.