1.Line a rectangular slow cooker with two layers of baking paper; do not preheat.
2.Finely chop 200g (6½oz) of the chocolate; cut remaining 100g (3oz) into chunks.
3.Microwave chopped butter in a large microwave-safe bowl
until melted; stir in sugar. Microwave for a further 1 minute or until very hot. Stir in the finely chopped chocolate until chocolate melts. Whisk in egg in two batches; stir in flour.
4.Pour batter into lined slow cooker. Smooth surface to remove any air bubbles. Scatter with reserved chocolate chunks and walnuts.
5.Heat slow cooker on LOW. Cover slow cooker insert with a clean tea towel, then replace cooker lid; cook for 3 hours. Turn off cooker; leave brownie to cool for 20 minutes in cooker, covered.
6.Transfer brownie to a cutting board; cut into 24 pieces.
Use macadamias, pecans or almonds instead of walnuts, if you prefer.
Store brownie in an airtight container, covered, in the refrigerator for up to 4 days. Warm in a microwave before serving, if you like.