Preheat the oven to 350°F (177°C) convection. Prepare one large cookie sheet and an 8-inch round pan by lining them with parchment paper.

In a stand mixer bowl, beat the egg whites at full speed until frothy. Gradually add 150 grams of granulated sugar while on medium speed, increasing to full speed until the meringue reaches stiff peaks.

Add the egg yolks and vanilla extract to the meringue, mixing at medium-high speed until fully incorporated.

Sift in the flour and salt while mixing on medium-low speed, ensuring not to overmix. Use a spatula to scrape down the bowl.

Transfer the batter into a large piping bag fitted with a round tip and pipe a thin layer into the prepared 8-inch pan. Bake for about 18 minutes or until golden and no longer wet.

Using the remaining batter, pipe straight lines onto the cookie sheet similar to éclairs. Sprinkle with granulated sugar and bake for 12 minutes until lightly golden.

Heat the milk in a saucepan until simmering. Whisk the egg yolks and 100 grams sugar in a medium bowl. Gradually add half of the hot milk to temper the eggs, then return to the saucepan and whisk consistently until it slightly thickens (180°F – 185°F).

Pour the custard into a bowl, adding butter and vanilla, mixing until combined.

Mix the gelatin with cold water and let it sit. Heat briefly until dissolved, then add to the custard. Cover with plastic wrap and refrigerate for about 2 hours until somewhat set.

Dice the strawberries and mix with 25 grams of granulated sugar and elderflower liqueur or lemon juice to release their juices.

In another bowl, sift the powdered sugar and add heavy whipping cream, mixing until it thickens and can hold stiff peaks.

Whisk the chilled custard until smooth, then fold in whipped cream in three parts until well combined.

If you used a cake layer, poke holes in it and drizzle the strained strawberry juice on top. Arrange ladyfingers around the edges, then add half of the strawberries and half of the Bavarian cream. Repeat the layers and refrigerate the assembled cake for at least 8 hours or overnight.

Whip the final portion of heavy cream with powdered sugar and vanilla. Pipe it on top of the chilled cake, garnishing with whole strawberries.