In a medium saucepan, combine egg yolks, whole eggs, salt, sugar, lemon zest, and lemon juice. Whisk well to blend everything together smoothly.

Heat the mixture over medium heat, whisking constantly. After about a minute of simmering, you will notice the curd thickening up. Continue whisking until smooth and thickened.

Once thickened, strain the curd through a wire strainer or cheesecloth into a non-metal heat-safe bowl to remove any lumps and zest. Add butter and let it sit for a minute to melt, stirring until smooth.

Cover the curd with parchment paper laid directly on the surface, then refrigerate until cold and thickened.

Combine water, sugar, lemon juice, and peeled lemon zest in a saucepan. Add basil, if desired.

Let the mixture simmer over medium heat, then cool. Strain out the peels and set aside in a bowl. If using limoncello, stir it in after removing from the heat.

Chill your mixing bowl and whisk in the freezer for about 5 minutes. Pour the heavy whipping cream into the chilled mixing bowl and whip until stiff peaks form.

In a separate large bowl, combine half of the cooled lemon curd and all of the mascarpone cheese. Whisk until fully combined and smooth—avoid overmixing, or the mascarpone may become gritty.

Fold the whipped cream gently into the lemon curd mixture using a whisk or spatula, taking care not to deflate the whipped cream.

Get a large (approximately 8” by 11”) non-metal casserole dish ready. Briefly dip each ladyfinger into the lemon syrup, one at a time, before placing them evenly in rows inside the baking dish.

Spread half of the filling on top of the ladyfingers, ensuring it reaches the edges. Repeat the dipping process with the remaining ladyfingers and place them in a single layer on top of the filling.

Add the remaining filling and spread to the edge of the dish. Cover and refrigerate for at least 8 hours or overnight for the best flavor.

Before serving, spread the rest of the lemon curd on top, smoothing it to the edges. Slice or scoop with a spatula and serve with a garnish of lemon slices for a refreshing finish.