To make the lemon curd: Bring a pot of water to a simmer over medium-low heat. Combine the egg yolks, sugar, lemon juice, and zest in a metal or glass heat-resistant bowl and whisk until smooth. Set the bowl over the simmering water, without letting the bottom touch, and continue to whisk. Keep working-out that arm and whisk it vigorously for a good 10 minutes, until the curd has doubled in volume and is very thick and yellow. Don't let it boil. Remove the bowl from heat and whisk in the butter, a couple of chunks at a time, until melted. Refrigerate until the custard is cold and firm.
To build the trifle: put the berries in a mixing bowl and toss them together so they are evenly distributed. Fold the whipped cream into the chilled lemon curd to lighten it up into a mousse. Line a glass trifle bowl with pieces of pound cake to fit. Drizzle or brush the cake with the Lemoncello, spoon a layer of the lemon curd over the cake, and then a layer of mixed berries. Repeat the layers until the ingredients are used up, the last layer looks best if it's the berries.