Preheat your oven to 170C / 350F (190C / 375F for no fan). Spray a 22cm / 9-inch square baking tin with oil spray and line the bottom and sides with baking paper.
Heat sugars and butter - Add the brown sugar, white sugar, salted butter and chopped chocolate to a heatproof bowl and microwave for 30 seconds at a time. Use a whisk to stir each time until melted and smooth.
Whisk in eggs - Add the eggs and egg yolk and whisk before adding the flour, cocoa powder, vanilla extract and walnuts (optional) and mix.
Bake - Pour into your prepared baking tin and sprinkle with chopped chocolate. Bake for 25 minutes. Allow to cool down to room temperature then place in the fridge for 2 hours to chill. This will make slicing it up easier. Use a large, sharp knife to slice into 3 x 3 squares. Serve chilled or at room temperature.
Dark Cooking Chocolate - Using good quality chocolate is important in this recipe. I used semi-sweet / dark cooking chocolate. This is different to eating chocolate and is specially made for baking with. You can find cooking chocolate in the baking isle of your supermarket.Why the egg yolk?The egg yolk helps make these brownies richer and chewy. It’s a remarkable thing and works with cookie recipes too!StorageThese brownies can be stored in an airtight container for up to three days. You can even freeze them. They don’t freeze solid, they just get super chewie. So delicious!How to stop your baking paper from sticking to the sticky brownie batter as it bakesIt’s simple! Just use little metal clips. I bought mine from amazon. But you can find them anywhere. Just make sure they’re not coated in paint. The plain shiny metals ones are best.Nutrition - is an approximate and is based on per bar. This recipe makes 9 bars.A note on measurements - this recipe is offered in weight measurements. This is in an effort to offer the most accurate way to get the best results. Cup measurements simply aren’t accurate enough for a recipe that needs precision and cup sizes differ depending on where you are in the world. Here is where you can learn more about how to measure ingredients using digital kitchen scales!