• Preheat the oven to 180C (350F). Butter a 10-inch(26 cm) springform pan (with a removable base), and dust with flour.  

Prepare the walnut sponge cake.

• In a large bowl, cream together butter and sugar until well mixed. Add walnuts, lemon zest and flour and mix well. 
• Separate egg yolks form egg whites. Whip egg whites until foamy and form stiff peaks. Fold in egg whites into butter mixture.
• Pour the butter mixture into prepared pan and bake for 15 minutes.

Prepare the yolk mixture.

• Meanwhile, mix egg yolks with sugar until thick, creamy, and pale yellow colored. 
• Remove the cake from the oven and add the yolk mixture on top of the cake and bake for another 15 minutes.

Prepare the chocolate glaze.

• Melt the chocolate over a bain-marie until smooth. Stir in butter and heavy cream until well blended.
• Use immediately, and pour over the cake.