Preheat the oven to 190 C / Gas 5. Line a shallow cake tin with parchment or tin foil and grease lightly to prevent the brownies from sticking.
Put the chocolate and butter in a heatproof bowl. Rest over a saucepan filled with a little bit of water. Heat up on a stove on low heat. Keep stirring frequently until chocolate melts.
In the meantime, place eggs, sweetener, flour and baking powder in a different bowl. Mix all together.
Once the chocolate is melted pour it in to the bowl with other ingredients. Mix all together. Add walnuts (or fruit) and stir. Pour the mixture in to the prepared cake tin and spread in to the corners.
Put in the oven and bake for 15 minutes.
Enjoy with whipped cream, chocolate sauce or on its own!
You can use dark or milk chocolate for this recipe. The 100g chopped walnuts can be substituted for dried fruit.