Let me show you a bulletproof way on how to make the best brownies over and over again. This recipe was given down from my grandmother, just that I left out the walnuts in this one which I recommend you should add in yours.

Useful baking information:
Water bath
For a water bath preheat a pot with water and place a glass bowl on top so that only the bottom of the glass bowl is in the water and can slowly heat the chocolate
Sugar mixture
If you don't have brown sugar you can also stay with only regular sugar but the 450g instead
Ingredients:
400g Baking chocolate with 40% cacao
250g butter
200g walnuts (optional)
400g sugar
80g brown sugar
1 tbsp vanilla sugar
1 tsp salt
1 tbsp maple syrup
150g flour
50g cacao powder
6 eggs
Lets start cooking:
Let's start by cutting the chocolate and the butter into very small cubes so that it will be easier to melt it
In a water bath melt both ingredients together until smooth

You must keep stirring during the process
When the mixture is smooth set aside to cool
In the meantime add all the sugars into a bowl and slowly whisk in one by one the eggs until you have a creamy texture
Mix in the maple syrup and the salt
Slowly under constant whisking add in the chocolate-butter mixture until you don't feel and granulated sugar anymore
In a bowl throw together the flour and cocoa and sift it into the bowl

With a spatula start folding in the flour and cocoa until it is fully incorporated
We are not going to whisk it as we want to keep the airiness
(Here is the option that you can add in some chopped up walnuts to give it some crunch in between)
In a 35x28 cm baking form spread out a baking sheet and brush it our with either a neutral oil or melted butter
Pour the batter into the baking pan and lightly tap it on the counter so that large air bubbles come to the top
Place the form in the preheated oven at 200°C for 15-20 minutes, the top should be light brown just like the picture

Let it cool for at least an hour or serve it warm with a scoop of ice cream
When cutting the brownies, slightly butter your knife and it will go much easier or heat the knife in hot water
Enjoy!