You can add any type of nuts or even chocolate chips into this recipe.
When double-boiling, do not let the bottom of the bowl touch the boiling water underneath. Let the steam of the boiling water heat the bowl and melt the chocolates in them.
Unlike any other cake, you don't want to have a clean toothpick when you test the center of the cake. If it happens, it means that you have over baked the brownies and it will be dry and hard on the inside. Brownies should be moist and a bit clumpy.
All the best! :)