- Preheat the oven to 180°C/160°C fan/Gas 4. Grease and line a 30 x 23cm cake tin with baking parchment.
- Put the chocolate and butter into a large heatproof bowl set over a pan of gently simmering water (don’t let the bottom of the bowl touch the water underneath) and leave until melted, stirring occasionally (see Tips).
- Remove from the heat and cool the chocolate down by whisking in the sugars using a balloon whisk. Add the eggs and vanilla extract and mix in with a wooden spoon until combined. Sift over the flour and salt and stir again until smooth and combined. Pour into the prepared tin, smoothing the top level.
- Bake for 35–40 minutes, until cracked around the edges and soft in the middle (it will firm up as it cools), rotating the tin halfway through baking.
- If you are adding the chocolate chunks, take the brownie out of the oven after 20 minutes and vigorously throw the chunks of chocolate at the half-baked brownie so they break the surface. Return to the oven for the remaining cooking time.
- Transfer the brownie to a wire rack and leave it to cool in the tin, then cut into squares before serving. Once cooled, the brownies will keep in an airtight container for up to a week. Alternatively, cool the cooked brownie in the tin for 10 minutes, then cut into squares and serve warm with a dollop of thick cream.
If you prefer, put the chocolate and butter into a heatproof bowl, cover (leaving a small air gap) and melt in the microwave on high for about 3 minutes, stirring occasionally. These brownies also freeze well for up to a month. Simply wrap them tightly in cling film and freeze, then thaw at a cool room temperature before serving.