Beat the butter, sugar, salt, oil and cocoa powder until it becomes lighter in colour and looks very creamy. Ideally, you want to use an electric mixer for this but it could also be with a whisk. If you're using a stand mixer, make sure to stop every once in a while to scrape down the sides and bottom of the bowl with a spatula.
Add the eggs and beat once again until fully incorporated and creamy once again. I remind you that if you're using a stand mixer you need to pause to scrape down the sides and bottom of the bowl with a spatula to incorporate everything nicely.
Add the milk and vanilla and beat to mix. You will notice that the mix curdles, but that's completely normal and comes back with the flour.
Add the flour and baking powder and beat to mix them in. Don't mix more than necessary though or your cake could turn out to be tough.
Add the chocolate chips using a spatula.
Butter up your pan with room-temperature butter. Make sure that you're spreading a thin layer that goes into all the corners of the pan.
Sprinkle flour in the pan and tap ti as you rotate it to spread it evenly. Finish by turning it upside-down with a good tap so that it releases the excess flour.
Pour the batter in the pan and bake it in a preheated oven at 180°C / 350°F for 40min – 1 hour. IF your oven is electric, bake it on top and bottom heat like always but check after 20-30 minutes to check if it's golden enough on top. If it is, switch to bottom only heat.
It will be ready when you can insert a skewer or pointy knife and it comes out without any raw dough. Remember that you could see a bit of melted chocolate from the chocolate chips and that's completely normal.
I like to remove my cake from the pan as soon as it comes out of the oven :).