Preheat the oven to 180C/350F/Gas 4.
In a bowl, whisk together the eggs, sugar and coffee for 7-8 minutes, or until lighter in colour, thick and well combined. Set aside.
Bring a little water to the boil in a pan, then reduce the heat until the water is simmering. Suspend a heatproof bowl over the water (do not allow the base of the bowl to touch the water). Add the butter and plain chocolate and stir until melted, smooth and well combined.
When the chocolate and butter have melted, remove the bowl from the pan and set aside to cool for 4-5 minutes.
When the chocolate and butter mixture has cooled slightly, stir in the egg and coffee mixture until well combined.
Carefully fold the flour, baking powder and salt into the batter mixture until well combined.
Carefully fold the nuts and white chocolate into the batter mixture until well combined.
Line 1-2 very shallow baking trays with greaseproof paper. Spoon dollops of the batter mixture onto the greaseproof paper, leaving a generous space between each spoonful of batter (the brownie biscuits will expand during cooking).
Transfer the brownie biscuits to the oven and bake for 10 minutes, or until the brownie biscuits have risen but are still soft in the centre and the tops have cracked slightly. Set aside to cool and harden on a wire rack before serving.