Preheat the oven to 180°C/Fan 160°C/Gas 4. Line two baking trays with baking paper. In a small saucepan, melt the butter over low heat, add the chopped chocolate and stir until melted. Set aside and allow to cool slightly.
In a bowl, sift together the flour, baking powder and salt. Use an electric mixer fitted with a whisk attachment to whisk the eggs on medium-high until foamy and lightly thickened, approximately 40 seconds.
Increase the speed to high and gradually add the sugar and the vanilla. Whisk until thick and pale. Reduce the speed and beat in the melted chocolate and butter until fully incorporated.
Change to a paddle attachment and on a low speed, gradually add the sifted flour mix, then the coarsely chopped chocolate, pecans and walnuts. Mix until evenly distributed.
Roll tablespoons of the mixture into balls and place on the prepared trays 4 cm (1½ inches) apart.
Bake for 17-20 minutes, until the biscuits are set around the edges, the centre will still be underdone. Do not over-bake. Leave to cool completely on the trays. Note: these biscuits can be stored in an airtight container for up to 3 days.