Preheat the oven to gas 4, 180°C, fan 160°C. Grease a 20cm baking tin and line with nonstick baking paper.
Put the butter and 150g chocolate in a microwave-safe bowl. Kids will love helping with this. Microwave on medium power for 1 min. Give the mixture a good stir, then continue to heat in 20-sec bursts, until the mixture has melted. Remove and set aside to cool a little.
Put the sugar, eggs and vanilla (if using) in a separate bowl and briefly whisk to combine. Stir in the cooled chocolate mixture – kids can help with this. Ask your little helpers to sift in the flour and cocoa, then you can add the remaining chocolate and mix well.
Pour the mixture into the prepared tin and bake for 20-25 mins, until a thin crust forms on the surface with a slight wobble.
Remove from the oven and leave to cool in the tin for 30 mins, before transferring to a wire rack to cool completely. Cut into 36 squares and chill until ready to serve. Enjoy cold from the fridge, at room temperature of warmed in the microwave for 10 secs. To freeze the brownies, wrap tightly in foil and eat within 2 month. Defrost for 1 hr at room temperature.