Line a 20 cm/8 inch square cake pan with baking parchment, making sure that it overhangs either side so that it is easy to lift the baked brownies out of the pan. Preheat the oven to 180°C/160℃ fan/350°F/gas mark 4.
Roughly chop half of the nuts, not too small, and set aside.
Place the butter, milk and sugars in a pan over a low heat and stir until melted and the sugar has dissolved; the mixture should be hot but not yet boiling.
Add the chopped chocolate to the pan and stir until completely melted then whisk in the rum and vanilla.
Sift together the flour, cocoa powder, cornflour, baking powder and salt then stir it into the chocolate mixture until no pockets of flour remain. Stir in the chopped nuts.
Pour the batter into the prepared pan and spread it level. Scatter (or arrange carefully) the whole nuts on top.
Bake for about 20-30 minutes until the brownie feels firm to the touch and a skewer comes out gunky but not wet. If it comes out clean the brownies are overbaked so keep an eye on them. You may need to loosely cover the top with tin foil partway through baking to stop the nuts from catching.
Leave the brownie slab to cool completely in the pan (I like to put it in the fridge overnight once it has cooled so that the brownies are easy to slice neatly), then carefully lift it out using the overhanging parchment and use a sharp knife to cut it into squares.