In a medium heatproof bowl combine egg yolks, lemon juice, limonchello, lemon zest and half of the sugar. Whisk the ingredients over a pan with shimmering water (double boiler) The bowl should not touch the water so make sure the pan is filled only at the bottom and the bowl is large enough to stay on the top.
Whisk for about 10 minutes, scrapping often the sides of the bowl. When the mixture starts thickening, remove the bowl from the heat and set aside to cool (15-20 minutes).
Meanwhile in another heatproof bowl combine the remaining sugar with the egg whites, salt and citric acid. Beat with an electric mixer (or just whisk) over the same pan with simmering water for about 5 minutes, until thick and stiff. Remove from heat and let cool (15-20 minutes).
In a separate chilled bowl whip the cold heavy cream for 3 to 5 minutes until stiff.
Combine the yolk mixture with the whipped cream, carefully folding with a rubber spatula. Gently fold in also the egg whites.
Pour half of the semifreddo mixture into a loaf or a rectangular pan leaned with cling film. Warm up the chocolate spread and spoon it on top, creating thin lines.
Pour the rest of the semifreddo on top of the chocolate and cover the pan tightly with cling film. Freeze for 6 to 8 hours.
Before serving let the semifreddo stay out of the freezer for about 10 minutes. Cut it in slices or make scoops.
Store the semifreddo for up to 1 month in the freezer.