Warm the milk with the lemon zest in a saucepan over medium heat until it just begins to steam. Do not boil.

Whisk the egg yolks and sugar in a bowl until pale, about 2 minutes. Sift in the cornstarch and whisk until smooth.

Pour the warm milk slowly over the yolk mixture, whisking constantly. Return the mixture to the saucepan over medium-low heat.

Stir continuously with a wooden spoon or silicone spatula until the cream thickens and holds a line on the back of the spoon, about 4 to 5 minutes.

Remove from heat. Stir in the lemon juice and cold butter cubes until the butter melts and the cream is smooth.

Pass through a fine-mesh sieve into a clean bowl. Press plastic wrap directly onto the surface to prevent a skin forming. Refrigerate for at least 3 hours until completely cold.

Heat the oven to 180 C / 355 F. Lightly grease the half-sphere silicone mold cavities.

Place the eggs, sugar, and salt in a heatproof bowl set over a saucepan of barely simmering water. Whisk constantly until the mixture reaches 40 C / 104 F and feels warm to the touch.

Remove from heat and beat with an electric mixer on high speed for 8 to 10 minutes until the mixture is pale, thick, and has tripled in volume. It should fall from the beater in a thick ribbon that holds for 3 seconds.

Add the lemon zest. Sift one-third of the flour over the foam and fold in with a large spatula using wide, slow strokes from the bottom of the bowl. Repeat with the remaining flour in two more additions.

Drizzle the cooled melted butter down the side of the bowl and fold in with two or three strokes. Stop as soon as no butter streaks remain.

Spoon the batter into the dome molds, filling each to the top. Tap the mold gently on the counter to release air pockets.

Bake for 15 to 18 minutes until the tops are golden and a skewer inserted in the center comes out clean.

Cool in the mold for 5 minutes, then unmold onto a wire rack flat-side down and cool completely.

Once the domes are fully cold, turn them flat-side up. Use a small melon baller or teaspoon to hollow out each dome, scooping from the center and leaving a 5 mm wall on all sides. Reserve the scooped crumbs.

Brush the inside of each hollow with the lemon syrup. Let it soak in for 2 minutes.

Whip 120 ml of the cold cream with the icing sugar to firm peaks. Fold the cold pastry cream until smooth, then gently fold in the whipped cream in two additions. Keep 4 to 5 tablespoons of this diplomat cream aside for the glaze.

Fill each hollow generously with the diplomat cream using a piping bag or small spoon. Smooth the flat base flush with a spatula. Refrigerate the filled domes for 1 hour.

For the glaze, whip the remaining 80 ml cold cream to soft peaks. Fold into the reserved diplomat cream until pourable but thick. If too stiff, add 1 teaspoon cold milk and stir.

Place each dome flat-side down on a wire rack over a tray. Pour the glaze over each dome in one slow, steady pour, covering the curved surface completely. Let any excess drip off.

Refrigerate the glazed domes uncovered for 30 minutes to set the glaze before plating.