Preheat oven to 180°C (350°F). Grease and line a 2lb loaf pan 23 x 14cm (9 x 5.5-inch) with parchment paper. Leave an overhang on the sides so that it makes it easier to lift the blondies out afterwards.
In a large bowl combine the melted butter and sugar and stir together. Add the egg and vanilla extract and stir again.
Sift the flour, baking powder and salt into the sugar mixture bowl and stir until just combined.
Add chopped chocolate and walnuts, reserving some of each to sprinkle on the batter just before baking. Fold the chocolate and nuts into the batter. Pour batter into your prepared loaf pan and sprinkle with remaining chocolate chunks and walnuts, pressing gently into the batter.
Bake for 22-25 minutes until set around the edges and with a slight jiggle in the centre. This ensures fudgy centres. Cook for 5 more minutes if a more set centre is preferred. Remove from the oven and allow to cool in the pan for ten minutes. Gently use the baking paper sling to lift the blondies out of the pan and onto a wire baking tray to cool before cutting.
Sprinkle with flaked sea salt before serving.