Preheat oven to 170C/340F.Line an 8in square baking pan with parchment paper.Add the chocolate and butter into a heatproof bowl.Place the bowl over a small saucepan of simmering water and gently stir until all the chocolate has melted.Remove from the heat and add the erythritol and baking powder.Add the eggs one at a time and mix well.Add the macadamia nuts and mix through.Pour the mixture into the baking pan and smooth out.Bake for 15-20 minutes until an inserted skewer comes out clean.Leave to cool for 20 minutes before cutting into 16 pieces.Enjoy warm or place in the fridge for an extra fudgy brownie.