Heat oven to 350℉/177℃.
Line 2 sheet pans with parchment paper. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and apple pie spice. Set aside.
With an electric mixer, beat butter and sugar until light and fluffy, about 2-3 minutes.
Add egg, egg yolk, and vanilla.
Beat until fully incorporated.
Add flour mixture.
Mix only until all ingredients are moistened.
Stir in oats, apples, and walnuts.
Drop by rounded spoonfuls onto prepared pans.
Flatten slightly.
Bake for 14 minutes, or until done.
Allow to cool on pan before removing.