Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan or line it with parchment paper.

In a large mixing bowl, combine the applesauce, brown sugar, granulated sugar, vegetable oil, eggs, and vanilla extract. Whisk until smooth, about 2-3 minutes.

In another bowl, whisk together the all-purpose flour, cocoa powder, baking soda, salt, and ground cinnamon.

Fold the dry mixture into the wet ingredients until just combined, keeping the batter slightly lumpy.

Fold in the diced apples and optional chopped nuts, reserving some apple pieces for topping if desired.

Pour the brownie batter into the prepared baking pan and spread it evenly using a spatula.

Bake in the preheated oven for 25-30 minutes, checking for doneness with a toothpick.

Once baked, cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.