Preheat your oven to 350°F (175°C). Lightly grease your 9x13-inch baking pan or line it with parchment paper, leaving some overhang for easy removal.
In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth. Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract and maple syrup until fully combined.
In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking powder, and salt. This ensures there are no lumps and the cocoa powder is evenly distributed.
Gradually add the dry ingredients to the wet mixture, folding gently with a spatula. Be careful not to overmix; just combine until no streaks of flour remain.
Fold in the chopped walnuts and chocolate chips evenly throughout the batter. This will create bursts of crunch and molten chocolate in every bite.
Pour the batter into the prepared pan and spread it out evenly. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted near the center comes out with moist crumbs (but not wet batter).
Remove the brownies from the oven and let them cool completely in the pan on a cooling rack. Once cool, lift them out using the parchment paper overhang and cut into squares.