Preheat the oven to 180°C (350°F). Line a 20x30 cm (or similar) baking pan with parchment and grease the sides lightly.
Make the sponge: in a large bowl whisk the Eggs together with Granulated Sugar and a pinch of Salt until the mixture is pale, thick and forms a ribbon when the whisk is lifted. Stir in the Vanilla Extract. Gently fold in the sifted All-Purpose Flour in two additions, taking care not to deflate the batter. Fold in the melted and cooled Unsalted Butter just until incorporated.
Pour the batter into the prepared pan, smooth the top and bake until golden and springy to the touch (about 12–18 minutes, depending on thickness). Allow the sponge to cool completely in the pan, then trim any high edges if desired.
Prepare the agar syrup: sprinkle the Agar-Agar Powder over the Water and let it hydrate for a few minutes. Combine the hydrated agar with a little extra Granulated Sugar in a small saucepan and bring to a gentle boil, stirring until the agar and sugar are fully dissolved. Simmer 1–2 minutes, then remove from heat.
Whip the Egg Whites with a clean pinch of Salt to soft peaks. With the mixer running on medium, slowly pour the hot agar-sugar syrup in a thin stream into the whipped whites to temper them (be careful—pour gradually). Increase speed and continue whipping until the mixture is glossy, thick and cooled to room temperature.
Flavor and enrich the filling: beat the cooled meringue briefly with a small pinch of Citric Acid and the Vanilla Extract. Then fold in the Sweetened Condensed Milk and beat or fold in softened Unsalted Butter in small pieces until the filling is smooth, stable and spreadable. Chill briefly if it needs to firm up.
Assemble the cake: spread the filling evenly over the cooled sponge in the pan, smoothing the top. Refrigerate for at least 1–2 hours (or until the filling is set) to make glazing easier.
Make the glaze: gently melt the Dark Chocolate with a tablespoon or two of Unsalted Butter over a double boiler or at low heat, stirring until smooth and glossy. Let the glaze cool slightly but remain pourable.
Finish and serve: pour the chocolate glaze over the chilled cake, tilting to coat the surface evenly. Return to the refrigerator until the glaze is set. For neat slices, warm a knife under hot water, wipe dry, and slice. Serve chilled.
