In a medium saucepan, whisk together egg yolks, granulated sugar, lemon juice, and lemon zest until smooth. Cook over medium-low heat, whisking constantly, until the mixture thickens enough to coat the back of a spoon, 6 to 8 minutes. Do not let it boil or the eggs will scramble.

Remove from heat and immediately whisk in the cold butter pieces until melted and fully incorporated. Strain through a fine-mesh sieve into a bowl to remove any cooked egg bits. Press plastic wrap directly onto the surface of the curd and refrigerate until completely cold, at least 1 hour.

In a large bowl with an electric mixer, beat the cold heavy cream, powdered sugar, and vanilla extract on medium-high speed until soft peaks form, 2 to 3 minutes. The cream should hold a peak that gently folds over at the tip.

Remove the chilled lemon curd from the refrigerator and whisk briefly to loosen. Gently fold about one-third of the whipped cream into the curd to lighten it, then fold in the remaining cream in two additions. Use a light hand and stop when no white streaks remain.

Spoon the mousse into 6 small glasses or ramekins. Refrigerate uncovered for at least 2 hours until set and chilled through.

Top with fresh berries and a pinch of lemon zest just before serving. The mousse will keep covered in the refrigerator for up to 2 days.