Prepare Oven and Pans:

• Preheat oven to 300°F (150°C).
• Line the bottom of an 8×3-inch round cake pan with a circle of parchment paper. Do not grease the sides of the pan. This helps the cheesecake climb up the sides.
• Prepare a 9×9-inch (or larger) baking pan for the water bath. It needs to be large enough to comfortably hold the 8-inch cake pan.

Heat Water: Bring a few inches of water to a simmer in a large saucepan or pot (this will be used for both melting ingredients and the water bath). Once simmering, reduce the heat to low.

Make the Base Batter:

• In a large heatproof bowl (one that can safely sit over a pot of simmering water without touching the water), combine the softened cream cheese, butter pieces, and heavy cream (or milk).
• Place the bowl over the simmering water (make sure the bottom of the bowl doesn’t touch the water). This creates a double boiler or bain-marie.
• Whisk the mixture constantly until the cream cheese and butter are completely melted and smooth, and the mixture is well combined.
• Remove the bowl from the heat.
• Add the room temperature egg yolks to the warm cream cheese mixture and whisk until well combined. Work swiftly to prevent the egg yolks from scrambling.
• Immediately sift the cake flour directly into the cream cheese mixture. Whisk until no dry flour or lumps remain.
• Remove the bowl from the pot and stir in the lemon zest (if using) and vanilla extract (if using). Whisk to combine.
• Set the mixture aside to cool slightly while you prepare the meringue.

Preheat Water Bath: Fill the 9×9-inch baking pan with the hot water (from the pot you used earlier, or freshly heated water) to about halfway full. Carefully place this pan in the preheated oven on the lower rack.

Prepare the Meringue:

• In a clean and completely grease-free mixing bowl (metal or glass is best), add the room temperature egg whites and the lemon juice.
• Using a hand mixer (or a stand mixer with the whisk attachment), whip the egg whites on medium-high speed until completely frothy (like bubble bath).
• Gradually add the granulated sugar to the frothy egg whites, about 1/3 at a time, while the mixer is still running. Add the sugar before the egg whites reach the soft peak stage.
• Once all the sugar has been added, continue whipping for another 10-20 seconds. Then, reduce the mixer speed to medium.
• Continue whipping the egg whites until they reach medium-soft peaks. The peaks should hold their shape but still curl over slightly at the tip. Do not overwhip to stiff peaks.

Combine Meringue and Batter:

• Add about 1/3 of the meringue to the cream cheese mixture. Gently fold the meringue into the batter until no streaks remain. This lightens the batter.
• Incorporate the remaining meringue in two more additions, folding gently after each addition until just combined. Do not overmix.

Pour and Bake:

• Pour the cheesecake batter into the prepared 8-inch cake pan.
• Firmly tap the cake pan on the counter a few times to release any large air bubbles. You can also use a skewer or toothpick to pop any visible bubbles.
• Carefully place the 8-inch cake pan (with the batter) inside the 9×9-inch pan of hot water that’s already in the oven.
• Bake for 80-90 minutes, or until the cheesecake is golden brown on top and a skewer inserted into the center comes out clean (or with just a few moist crumbs). The cheesecake should still have a slight jiggle in the center.

Cool in Oven:

• Turn off the oven. Leave the cheesecake in the oven with the door closed for 30 minutes.
• After 30 minutes, crack open the oven door slightly (leave it ajar) and let the cheesecake continue to cool slowly in the oven for another 30 minutes. This gradual cooling process helps prevent cracking.

Cool Completely:

• Carefully remove the cheesecake from the oven and the water bath.
• Dry the bottom of the cake pan if any water got on it.
• Let the cheesecake cool for a few minutes on its side, gently rotating the cake pan every 20-30 seconds. This helps the cheesecake release itself from the sides of the pan.

Remove from Pan: Carefully invert the cheesecake onto a plate. Remove the parchment paper from the bottom. Flip the cheesecake upright onto another plate. Let it cool completely at room temperature.

Chill (Optional but Recommended): While the cheesecake can be enjoyed warm, it’s even better after chilling. Chill in the refrigerator for at least 1 hour (or longer, for a firmer texture) before serving.

Serve: You can serve the Japanese cheesecake plain, dust the top with powdered sugar, or brush the top with slightly warmed apricot jam for a glaze. Serve with fresh berries and/or sweetened whipped cream, if desired.