In a saucepan over medium heat, melt the butter with the chocolate. I used a chocolate with almonds and hazelnuts, but if you use a plain one (which I recommend), don't add the nuts until the end (you'll see the step later).

When everything is melted and we have a homogeneous mixture, remove from heat and transfer to another container to cool.

When it has cooled (after about 5 or 10 minutes), add both types of sugar.

Beat with a hand whisk until well integrated.

Add the eggs one by one, beating after each addition. When adding the last egg, also incorporate the vanilla and the pinch of salt.

Add the sifted flour. For this, we can use a mesh strainer. It's very important to use all-purpose flour without any leavening agents.

Mix well with a spatula using folding motions.

Once the brownie batter is ready, add the chopped walnuts (or the nuts of your choice) and mix again to integrate them.

Grease a shallow baking pan. We can grease it with non-stick spray, or coat it with a little butter, margarine, or light olive oil.

Pour the mixture into the pan and bake for 30 minutes.

The brownie is ready when the surface looks crunchy and cracked, and the edges have pulled away from the pan. To be sure, you can insert a toothpick, and it should come out with some crumbs but not liquid, more like creamy. If necessary, leave it for 10 more minutes (or as long as needed). Remove from the oven and let it rest in the same pan for 10 minutes.

After this time, remove from the pan and let it cool completely on a rack or a plate, for about an hour to prevent it from breaking when cutting.

And well... Serve and enjoy this delight, it's exquisite! It can be served with a scoop of vanilla ice cream, with powdered sugar or melted chocolate on top... or just on its own!