Preheat the oven to 180C/350F/gas mark 4 and grease and line a 20cm square cake tin with baking parchment.
Place the chocolate and butter in a heatproof bowl and place it over a pan of gently simmering water, making sure that the base of the bowl doesn't come into contact with the water. Stir until the chocolate and butter have melted then remove from the heat and set aside to cool a little.
Sift together the flour, cocoa powder and salt and set aside.
Place the eggs and sugars in a large bowl and beat with an electric mixer on high speed until extremely pale, thick and mousse-like and about doubled in volume. If you lift the beaters out of the bowl and wiggle them from side to side, the mixture that falls off the beaters should leave a trail on the surface that lasts for a couple of seconds.
Pour the chocolate mixture into the bowl and gently fold it in with a rubber spatula until the two mixtures are just combined, be careful not to knock out the air.
Sift the flour and cocoa powder mixture over the top and fold it in carefully, the mixture will become quite gungy. Finally add the truffles and fold them through, reserving a few to scatter over the top if you like.
Pour the mixture into the prepared tin and level it, scatter over any reserved truffle pieces. Bake in the centre of the oven for about 35 minutes, though start checking after 25; when they are ready the brownies should be set in the middle - if they wobble when you shake the tray they need a little longer, and a skewer inserted into the centre should come out a bit gungy, with moist crumbs but no wet batter, if it comes out clean they are overdone. Keep a close eye on them when they are nearly ready as brownies can go from perfectly fudgy to dry and overbaked in a matter of minutes.
Slide the tin onto a wire rack and leave the brownies to cool completely, they will firm up as they cool and are near impossible to slice neatly when warm; I like to place the tin in the fridge overnight then carefully un-mould the slab and slice into 9 squares.
They will keep in an airtight container for about a week and freeze well. To serve, give them a 20-30 second blast in the microwave and top with a dollop of strawberry or vanilla ice cream.