Preheat the oven to 170°C/ 325°F/ gas 3. Grease and line a 20cm (8in) square brownie tin or shallow baking tray.
Place the chocolate and butter in a heatproof bowl over a small saucepan of simmering water.
Stir it frequently until it is just melted, making sure the water does not touch the bottom of the bowl. Remove from the heat and allow to cool.
Once the chocolate and butter is cool enough, whisk in the caster sugar and ground almonds.
Then add the egg yolks and beat until the mixture is smooth.
In a separate bowl, whisk the egg whites to form stiff peaks..
Whisk one-third of the egg whites into the chocolate mixture to loosen it, then gently fold in the remaining egg whites, taking care not to lose too much air.
Fold the pecans into the brownie batter, then pour it into the prepared tin.
Bake the brownie in the centre of the oven for 30-35 minutes, until the edges are set, and a toothpick inserted into the centre comes out with a little batter attached.
Leave the brownie to cool completely in the tin.
Now turn it out and cut into 12-16 equal pieces.
Dust with icing sugar or cocoa powder to serve if you prefer.