In a large mixing bowl, sprinkle the gelatin over the water and set aside.

In a medium mixing bowl, whisk the egg yolks and cornstarch until combined.

In a medium saucepan, combine the fresh or thawed berries, granulated sugar, unsalted butter, and salt. Mash and mix before turning on the heat. Cook over medium heat, mixing frequently until simmering and the fruit is almost completely broken down – about 4 to 6 minutes.

Remove the fruit from the heat. While whisking constantly, slowly stream about half of the fruit into the egg yolk mixture. Whisking constantly, return the tempered mix back to the saucepan. Over medium heat, continue to whisk and cook until the curd thickens and bubbles – about 1 minute.

Strain the curd through a fine-mesh strainer set over the bowl with the gelatin. Press through with a spatula until only the seeds remain. Discard the seeds and stir the curd with the gelatin until dissolved.

Set aside, stirring occasionally, until the curd is lightly thickened and cooled to room temperature – at least 30 minutes or up to 1 and ½ hours.

Adjust oven rack to middle position and preheat the oven to 350F degrees. Line the bottom of 2, 8-inch round cake pans with parchment paper and lightly grease and flour the pans.

On a piece of wax or parchment paper, sift the cake flour and salt. Set aside.

In a medium microwave-safe bowl, melt the unsalted butter and set aside.

In the mixing bowl of a stand mixer fitted with the whisk attachment, add the eggs and granulated sugar. Start mixing on low speed to break up the eggs. Increase speed to medium and mix until airy, pale, and tripled in volume (about 4 to 5 minutes). Add vanilla extract during the last moments of mixing.

Remove mixing bowl from the mixer. Sprinkle ⅓ of the sifted flour over the batter. Gently fold in the flour, stopping as soon as incorporated. Repeat this 2 more times.

Spoon about 1 cup of cake batter into the bowl of melted butter. Fold the butter into the batter and transfer this mixture back to the mixing bowl, gently folding until incorporated.

Pour the batter into the prepared cake pans and smooth the top. Bake for 25 to 27 minutes, until the tops are springy and a toothpick comes out clean.

Cool the cakes in the pans on a wire cooling rack for 10 minutes. Invert the cakes onto the wire rack to cool completely.

Clean the work bowl and whisk attachment for the stand mixer. Add heavy cream and mix until soft peaks form.

Transfer ⅓ of the whipped cream to the fruit curd and whisk gently. Add the remaining whipped cream and fold into the fruit until no streaks remain.

Add jam and lemon juice to a microwave-safe dish and microwave for about 30 to 60 seconds until hot and fluid.

Set one cake on a serving platter. Spread half of the jam and half of the berry mousse on top. Top with the second cake and spread the remaining jam and mousse on top.

Refrigerate the cake for at least 4 hours, or up to 24 hours, to allow the mousse to set. Serve at room temperature.