For the syrup and garnishing
Butter a square baking pan (23cm - 9inch side). Beat eggs with sugar in a bowl using a mixer until they white. Add the flour with the cinnamon and continue beating, then add the remaining butter and walnuts.
Pour the mixture into the pan and bake at 180°C (356°F) for 40'. Leave it in the oven and prepare the syrup as follows: in a small saucepan mix the water with the sugar, add the peel and juice of half a lemon and boil it for 7' to 8' minutes. Sprinkle the cake with the syrup, after you've removed the lemon peel.
Beat the cream with the mixer until it turns to thick whipped cream. Add caster sugar and continue beating. To serve, cut the walnut pastry into square pieces and garnish them with whipped cream.