In a bowl, sift together the Farina flour and cocoa powder. Add the salt and mix well.
In a saucepan, melt the butter over low heat. Once melted, add the couverture chocolate and stir until fully melted and smooth. Remove from heat and set aside.
In a separate bowl, or the bowl of a mixer, combine the eggs and both types of sugar. Whisk briskly until the mixture is frothy and the sugar is dissolved. Add the vanilla extract and mix well. Gradually pour in the melted chocolate-butter mixture, stirring until the batter is smooth and well combined.
Gently fold in the flour mixture using a spatula, mixing in circular motions to avoid overmixing and activating the gluten.
Line a 20 cm square baking pan with non-stick greaseproof paper. Pour the brownie batter into the prepared pan.
For the topping: Drop spoonfuls of crunchy peanut butter evenly over the surface of the brownie batter. Using the back of a spoon, create circular swirls in the mixture to partially incorporate the peanut butter into the chocolate batter. The peanut butter may be stiff at first, but it will soften as you work it.
Bake in a preheated oven at 170°C (340°F) for 35 minutes. Allow the brownies to cool completely in the pan before cutting them, as the chocolate will still be soft and gooey when first removed from the oven.