Preheat the oven to 350°F (180°C.). Grease a 9-inch (23 cm) square baking dish. Line with parchment paper, ensuring that each side has pieces that hang over the edges; set aside. Whisk flour with cinnamon and cayenne; set aside.
Place 4 oz (125 g) of chocolate, peanut butter and butter in heatproof bowl set over barely simmer water. Heat, stirring occasionally, until melted and smooth. Remove from heat. Stir in sugar. Cool slightly.
Whisk each egg into the chocolate mixture, one at a time, until well combined; stir in vanilla. Stir in flour mixture until combined. Scrape the batter into the prepared dish; smooth top, sprinkle with peanuts.
Bake in the centre of the oven for 20 to 25 min or until only a few moist crumbs stick to a toothpick when inserted into the centre. Cool completely in baking dish.
Melt remaining 1 oz (30 g) chocolate. Drizzle over top of brownies.
Using parchment paper as handles, remove brownies from dish. Peel off parchment paper. Cut into 16 squares.
Store, refrigerated, in airtight container for up to 3 days or freeze for up to 6 months.