Lightly grease 2 baking trays and line with baking paper.
Pour boiling water into a small saucepan off the heat, about a third full and sit a small bowl on top. Add chocolate pastilles and stir occasionally, until chocolate melts. Remove from heat and allow to cool for 10 minutes.
Meanwhile, cream butter, peanut butter, sugars and vanilla until light and fluffy. Add eggs one at a time, beating until combined. Add flour, cocoa, bicarbonate soda and salt, mixing until well combined. Add cooled melted chocolate and gently stir until combined. Fold in salted peanuts until just combined.
Using a spring loaded ice cream scoop with a 5.5cm diameter, place 10 scoops on each baking tray 5cm apart.
Place baking trays in refrigerator for at least 2 hours, to chill the mixture. This is very important so the brookies hold their shape and achieve a brownie texture in the middle, otherwise they will spread to flat biscuits.
Preheat oven to 180°C (160°C fan-forced).
Place chilled baking trays in preheated oven and cook for 15 minutes.
Remove from oven and allow brookies to cool on trays.