1. Preheat the oven to 180 degrees Celsius or 160 degrees if using a fan oven. Grease a 9 x 13-inch deep rectangular baking tin and line it with parchment paper, ensuring the paper comes up and over the sides of the tin.

2. Put the chocolate and butter in a heatproof bowl and gently melt together over a saucepan of simmering water, making sure the bottom of the bowl doesn't touch the water. Alternatively, melt the butter and chocolate together in the microwave, stopping every 30 seconds to mix. Once melted, leave to cool while preparing the rest of the ingredients.

3. Sieve together the flour, cocoa powder, baking powder and salt into a bowl and set aside.

4. Whisk the eggs, caster sugar and brown sugar together using an electric beater or stand mixer for two to three minutes until light and fluffy.

5. Slowly add the chocolate and butter and mix well to combine. Switch to the beater attachment and slowly add half of the dry ingredients, mixing well to combine, before adding the second half and mixing thoroughly.

6. Scrape the mix into the prepared tin and spread out evenly using a spatula or palette knife. Bake in the preheated oven for 25 minutes.

7. Check the brownies, they should be crisp and set around the outside with a very slight wobble in the centre.

8. Remove from the oven and leave to cool at room temperature for an hour or two before transferring to the fridge to cool completely. Portion into 12 squares. The finished brownies will keep for up to three days in a sealed container.