Preheat your oven to 350°F (175°C).

Grease a 9x13 inch baking dish or line it with parchment paper.

In a medium bowl, mix the all-purpose flour, baking powder, salt, and cinnamon together and set aside.

In a large bowl, combine melted butter, brown sugar, and granulated sugar until smooth. Add eggs and vanilla extract, beating until creamy.

Fold in diced apples until evenly distributed.

Gradually add the flour mixture to the wet mixture, stirring until just combined. Avoid overmixing.

If using, fold in chopped walnuts.

Pour the batter into the prepared baking dish and spread evenly.

Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Allow to cool in the pan for about 10 minutes before slicing into squares and serving.