Melt the butter in a sauce pan with low heat. Add the chocolate pieces into the butter and keep stirring until the chocolate is fully melted.
Take the sauce pan away from the burner and leave it on the counter.
Place the eggs and sugar in a large bowl. Mix it with a whisk for a minute.
Start adding the melted chocolate slowly into the egg-sugar mixture while you keep stirring.
Finally add the flour and vanilla on top and stir until everything is blended with each other.
Find a medium sized oven tray and oil it with a tablespoon of vegetable oil.
Then place a piece of baking paper on top of the oiled tray.
For the next step, pour one more tablespoon of vegetable oil on top of the baking paper and oil it thoroughly with your fingers.
Pour the brownie mixture into the oiled and paper-covered baking tray.
Preheat the oven at 150°C. (Do not use the fan mode!)
Bake the brownies in the oven for 30 minutes. This way the insides will be a little more liquid. But if you prefer it baked a little more, you can complete it to 35 minutes.
Take the tray out and leave it on the counter at least half an hour. This way the surface will crack even further.