1. Preheat your oven to 350°F (175°C). Grease a 9×13 inch baking pan or line it with parchment paper for easy removal later.
2. In a large mixing bowl, whisk together the melted butter, granulated sugar, and brown sugar until well combined.
3. Add the eggs and vanilla extract to the mixture, and continue whisking until smooth and creamy.
4. In a separate bowl, sift together the flour, cocoa powder, baking powder, and salt.
5. Gradually add the dry ingredients to the wet ingredients, stirring just until combined. Be careful not to overmix.
6. Fold in the chocolate chips and walnuts, ensuring they are evenly distributed throughout the batter.
7. Pour the batter into the prepared baking pan, spreading it evenly with a spatula.
8. Bake in the preheated oven for 25-30 minutes or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs.
9. Allow the browkies to cool in the pan for at least 20 minutes before cutting into squares.

Remember, patience is key here. Letting them cool will enhance their chewy texture.