Preheat the oven to 350°F. Grease a 9-inch round cake pan with 15 g unsalted butter and line the bottom and sides with parchment paper, including two strips crossed in an "X" for lifting. Fill a baking sheet filled with hot water (bain-marie) and place in the oven.
The Batter
Set up a double boiler with simmering water. In a bowl over the simmering water, combine 300 g cream cheese, 60 g unsalted butter, 200 ml whipping cream, and 60 g sugar, mixing until melted. Remove from heat and whisk in 6 egg yolks one at a time. Sift in 80 g cake flour, mix, and then strain the batter into a clean bowl. Add the 0.5 lemon zest and 30 ml lemon juice, whisking to combine.
In a separate bowl, whip 6 egg whites until foamy. Gradually add 100 g sugar and continue whisking until stiff peaks form. Gently fold a third of the whipped egg whites into the cream cheese mixture, followed by the remaining whites until just combined.
The Baking
Pour the batter into the prepared pan, then place the pan in the baking sheet filled with hot water (bain-marie). Bake for 70-75 minutes at 320°F, then lower to 300°F for another 10 minutes, checking for doneness with a skewer. Turn off the oven and leave the cake inside with the door slightly open for 15-20 minutes.
The Serving
Carefully lift the cake using the parchment straps, brush with a mixture of 2 Tbsp. apricot jam and 2 tsp. hot water, and let it cool to room temperature. Serve immediately or chill before serving.