Preheat the oven to 325 °F / 160 °C / Gas Mark 3, no fan (if you have a convection oven, reduce the temperature by 20°).
Prepare a square 8x8 inch / 21x21 cm cake pan (measured at the bottom), grease it lightly with butter and line with baking paper.
Melt the chocolate and butter using double-boiler method: place a heatproof bowl (glass or metaover the pot with boiling water (should be over low heat, and the bowl can't touch the water surface). Add the butter and chocolate to the bowl, stir until the chocolate mixture has melted.
Remove the bowl from the pot, wait for about 5 minutes until the mixture is a little cooler (but still warm), stir in sugar, until just combined.
Add the eggs, whisk thoroughly, until combined.
Add the sifted flour and cocoa, then the peanuts, quickly stir in until just combined.
Pour the batter into the prepared pan, spread it evenly.
Place the plum halves on top.
Bake for about 45-50 minutes, it's ready when a skewer inserted in the centre comes out with moist crumbs clinging (the brownie will continue to bake slightly in the hot pan once pulled out of the oven). The center of the brownie shouldn' be wobbly when shaken. Be careful not overbake it (it's overbaked when a skewer comes out clean). It's better to underbake a brownie that to overbake.
Set aside to cool completely (about 1-2 hours), then cut into squares.