Start by melting the chocolate in a bain-marie (i.e., hot water bath) or in a microwave and transfer it to a large bowl.
Add the brown sugar and stir it well.
Add the olive oil and stir it well until you reach a homogeneous mixture.
Add the powdered instant coffee and the eggs and whisk them well.
Add a pinch of salt and the vanilla essence and incorporate everything.
Add the raw cacao powder and incorporate it well.
And add the oat flour and once again, fold it well.
Add the chopped walnuts and mix them well into the batter.
Pour the batter into a square baking pan, lined with baking paper.
Gently tap the pan on a flat surface so that the air is released.
Bake in a preheated oven at 180º for 25 to 30 minutes or until half-baked on the inside.
Take it out from the oven and let it cool completely.
Sprinkle with cocoa powder and salt.