Melt butter with crushed dark chocolate in a bowl over a hot water bath, stir in sugar, salt and vanilla. Gradually stir in egg, then fold in flour and nuts.
Line a 28x28 cm / 11x11 inch pan with parchment paper.
Pour chocolate mixture into the pan, smooth surface and bake in preheated 180°C / 350°F oven for about 35 minutes. Bake until brownies start to pull from the edge of the pan. Remove from the oven and allow to cool in the pan. Lift out with the help of the parchment paper and cut into 3.5x3.5 cm / 1-1/2x 1-1/2 inch cubes. Store cool. Serve garnished with candied lemon peel and powdered sugar.