Step 1

Lightly sprinkle the inside of an 8½-by-4½-inch or 9-by-5-inch loaf pan with water. Line the pan with a 15-inch sheet of plastic wrap placed lengthwise in the pan. Press the plastic against the bottom, into the corners and up the sides of the pan; the water helps the plastic adhere. Smooth any wrinkles; allow the excess to overhang the edges of the pan.

Step 2

In a stand mixer fitted with the whisk attachment, mix the sugar and lemon zest on medium until the sugar is pale yellow and fragrant, about 15 seconds. Add the eggs and whip on medium until frothy, then gradually increase to medium-high and whip until lightened in color, doubled in volume and holds a soft peak when the whisk is lifted, 3 to 4 minutes. Gently transfer to a large bowl.

Step 3

In the now-empty mixer bowl (no need to wash it), combine the cream and creme fraîche. Using the whisk attachment, whip on medium-high until the mixture holds medium peaks, about 2 minutes. Reduce to low and drizzle in the vodka and lemon juice; beat on low until incorporated, about 15 seconds (the mixture will deflate slightly). Add the lemon curd and mix on medium just until combined, about 15 seconds.

Step 4

Scoop one-third of the cream-curd mixture into the whipped egg mixture. Using a silicone spatula, fold until just a few streaks remain. Fold in the remaining cream-curd mixture; it’s fine if a few streaks remain. Gently pour the mixture into the prepared pan and smooth the surface. Cover with another sheet of plastic wrap set directly against the surface. Freeze for at least 6 hours or up to 2 weeks.

Step 5

When ready to serve, peel off the plastic wrap covering the mousse. Make sure the plastic overhang is pulled to the sides and not covering the surface. Overturn a serving platter onto the loaf pan and, holding the two together, carefully invert. Lift off the pan, tugging at the plastic wrap, if needed, to unmold the mousse. Remove and discard the plastic wrap. Cut into slices and serve with fresh berries (if desired).