Preheat the oven to 180°C (350°F/Gas 4). Lightly grease and line a 20cm (8in) square cake tin with baking parchment, leaving some overhang. Melt the butter and both lots of chocolate in a heatproof bowl over a saucepan of simmering water, making sure it does not touch the water. Stir until smooth, then leave to cool.
Gradually add the vanilla extract and both lots of sugar to the mixture and whisk well to combine. Then add the eggs, one at a time, whisking well after each addition until smooth. Place the flour, cocoa powder, salt, and baking powder in a separate bowl and mix well.
Use a spatula to fold the dry ingredients into the chocolate mixture and combine until smooth. Then mix in both lots of chocolate chips until evenly incorporated. Pour the brownie mixture into the prepared tin, and spread it out evenly.
Bake for 40-45 minutes, until an inserted toothpick comes out clean. Leave to cool slightly before removing the brownie from the tin. Then cut it into nine equal-sized pieces, cleaning the knife with a damp kitchen towel between cuts. Serve warm with vanilla ice cream, if desired. You can store them in an airtight container for up to 5 days.
Make it vegan
Replace the unsalted butter with the same amount of vegetable oil spread. Mix 2 tbsp of flax seeds with 6 tbsp of warm water. Let it sit for 15 minutes and use the mixture in place of the eggs in step two. For the chocolate and chocolate chips, choose vegan varieties.