Preheat the oven to 350°F. Line a 9×9-inch metal baking pan with parchment paper and lightly spray with nonstick cooking spray.
Microwave the butter and chopped chocolate in 30-second intervals, stirring after each one, until melted and smooth. Cool for five minutes.
Whisk in the granulated sugar and brown sugar. Add the eggs one at a time, followed by the vanilla, whisking until glossy.
In a separate bowl, whisk together the flour, cocoa powder, salt, and cinnamon.
Toss the diced apple with one tablespoon of the dry mixture.
Fold the remaining dry ingredients into the chocolate mixture just until combined.
Fold in the apples and ½ cup of chocolate chips.
Spread the batter evenly into the prepared pan. Sprinkle the remaining ¼ cup of chocolate chips over the top.
Bake for 32 to 38 minutes, or until a toothpick inserted near the center comes out with moist crumbs but no raw batter.
Cool completely in the pan for at least two hours before lifting out and cutting into 16 squares.