Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper.

Wash and peel the apples, then chop them into small, bite-sized pieces and set aside.

In a large mixing bowl, combine melted butter and sugar. Stir until well blended, then add eggs one at a time, mixing thoroughly. Stir in vanilla extract.

In a separate bowl, combine flour, baking powder, cinnamon, and salt for even distribution.

Gradually add the dry mixture into the wet ingredients until just combined. Fold in the diced apples and optional nuts, being careful not to overmix.

Transfer the batter into the prepared baking pan, smoothing the top with a spatula.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out clean.

Once done, allow the brownies to cool in the pan for at least 10 minutes before slicing.