Line a greased
23cm/9in square baking tin with baking parchment.
Melt the chocolate with the butter in a large
heatproof bowl over simmering
water. Remove from
the heat and mix well. Mix in the sugar and salt.
Stir in the eggs and vanilla essence, blending well.
Fold in the flour until blended. Spread evenly into
the baking tin. Chill until cold for about 2 hours.
Preheat the oven to 180°C/350°F/Gas mark 4. About 10
minutes before baking, prepare the walnut topping by
melting the butter in a heavy-based saucepan over a
low heat.
Stir in both sugars and cook, stirring
constantly, for 1 minute. Remove from heat, stir in
the flour and eggs, blending well.
Return to the
heat and cook for about 3 minutes, stirring until
thick and light in color. Remove from the heat and
stir in the vanilla essence and walnuts, stirring
well to coat the nuts evenly.
Spoon over the chilled
bottom layer, spreading to the edges. Bake for 40 to
45 minutes. Cool on a wire rack before cutting into
slices.