Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, baking soda, and cinnamon.
In a large bowl, mix butter with peanut butter and brown sugar until fluffy. Add in egg and vanilla extract. Mix until smooth. Add the flour mixture in two parts, mixing well until a thick dough forms.
Fold in diced apples. Press the cookie dough into a greased 8-inch square baking dish. Bake for 25-30 minutes. Cut into bars.