Preheat oven to 325 degrees F. Line the bottom of a 10 inch spring form pan with parchment paper but do NOT grease the sides. Greasing the sides will not allow the cake to rise well and since the cake is cooled in the pan, the ungreased sides provide support for the cake as it cools so that it will not shrink. (See NOTE for 9 inch Springform Pan)

In the bowl of an electric mixer, using a whisk attachment, combine the sugar, eggs and vanilla extract and beat on medium high speed for about 10 minutes until the mixture is foamy and pale in color.

Reduce the speed of the mixer to medium low and with the mixer running, begin to continuously sprinkle in rounded tablespoons of the flour. Stop the mixer as soon as the flour is fully incorporated.

Remove 1 to 2 cups of the sponge cake batter and mix it with the melted butter.

Immediately add this mixture back into the main batter, folding it in very gently with a rubber spatula. Make sure the butter mixture is fully mixed in but be careful not to over mix the batter when folding as this can cause the batter to deflate.

Pour the batter into the prepared 10 inch spring form pan and bake for approximately 45 minutes or until the top springs back fully when pressed lightly. Watch this cake closely as it will over bake easily if left for 5 minutes too long. Start checking it at the 40 minute mark just to be sure, although it always takes the full 45 minutes in my oven.

Cool the cake completely IN THE PAN for at least a couple of hours before carefully and slowly running a sharp knife around the outside edge of the pan to release the cake from the sides. Be sure that you are holding the knife straight at a 90 degree angle, pressing it firmly against the side of the pan as you run it around the edge. This will ensure that you do not damage the sides and they will come out looking perfect in your presentation.

Release the sides of the spring form pan and peel the parchment paper off the bottom of the cake before placing it back into the spring form pan. The pan then acts as a mould to add the lemon mousse and allow it to set.

In a small saucepan combine the egg yolks, sugar, lemon juice and zest.

Cook slowly over medium low heat for about 10 minutes stirring constantly until the mixture thickens enough to coat the back of a wooden spoon.

Remove from heat and stir in the butter a few pieces at a time until completely smooth. Cover with plastic wrap and chill completely in the fridge.

Note: I don't mind leaving the finely minced lemon zest in my lemon curd because I think it develops even more flavour as it chills, but if you prefer it smoother, you can run it trough a fine sieve before chilling.

Whip the whipping cream, icing sugar and vanilla extract to firm peaks.

Using a rubber spatula, fold 1/2 of the chilled lemon curd through the whipped cream until thoroughly blended. (The other half of the lemon curd is reserved for the top of the cake.)

Once you have the sponge cake back in the spring form pan as directed, you can sprinkle it with the Limoncello liqueur if you are using it. (See NOTE for 9 inch Springform Pan)

Add the prepared lemon mousse to the pan and chill for several hours or overnight.

When the cake is well chilled and the mousse has set, run a small sharp knife around the edge of the mousse to loosen it from the pan.

Slide the cake from the bottom of the pan onto a serving plate. Spread the reserved lemon curd evenly over the cake.

Pipe the vanilla whipped cream around the border or just use rounded teaspoonfuls to dollop around the edge. Garnish with the candied lemon peel if desired.

Cut a large lemon in thick slices and then cut the slices in half. Remove the lemon flesh from the peel.

Bring the 3/4 cup sugar and water to a slow simmer and add the lemon peel.

Slowly simmer for 15-20 minutes, then remove the peels from the syrup and drain for a minute or two on a wire rack before tossing the sticky peel into the additional sugar until it cools.