Cake preparation: In a food processor, roughly grind walnuts. Set aside.
In a heat proof bowl placed on a saucepan with warm water but not simmering (the bowl can't touch the water), melt the chocolate with butter while stirring until the mixture is creamy (personnaly I would melt the chocolate and butter in the microwave). Let cool slightly.
In a bowl, with an electric mixer, beat eggs with the sugar at medium-high speed for 3 minutes or until the mixture is thick and foamy. Add salt and vanilla, then reserved walnuts and stir with a spoon. With a spatula, incorporate the chocolate mixture by folding delicately until the dough is homogeneous (don't stir too much). Spread the dough in a cake pan of 9 inches in diameter (that has parchment paper or is greased).
Cook in a preheated oven of 350 F for about 30 minutes or until a tooth-pick inserted in the center comes out clean. Put the pan on a rack and let cool for 10 minutes, then remove from the pan.
Sauce Preparation: Meanwhile, in a saucepan, melt butter. With a whisk, incorporate the flour. Add cream, sugar and salt and cook at medium heat, stirring, until the mixture has thicken and is simmering. Keep cooking, stirring, for 2 minutes. Remove the saucepan from the heat and incorporate walnuts and vanilla. Let cool slightly.
When ready to serve, cut the cake in 8 slices (like a pie) and put creamy sauce on top.