Preheat the oven to 160°C / 320°F.
Grease two 15cm baking pans and place baking paper on the base.
In a bowl sift the flour, cocoa powder, salt and baking powder. Mix briefly and set aside.
In the mixer bowl add the butter, sugars and vanilla.
Beat on high speed for about 5 minutes until the mixture looks creamy, pale and fluffy. (see notes 1)
Go through the bowl with a spatula to check that everything has been well incorporated.
Beat at medium speed and add the eggs one by one.
After incorporating the 3 eggs, go back through the bowl with the spatula.
Beat at minimum speed and gradually add the dry ingredients that we had reserved. (see notes 2)
Divide the mixture equally into the two prepared pans and bake in the preheated oven for approx. 25 minutes. (see notes 3)
Remove from the oven and let rest for 10 minutes in the moulds.
Unmold onto a wire rack and let cool completely to room temperature.