Preheat the oven to 375⁰F/190⁰C. Spray baking pan with removable bottom with cooking spray and line with parchment paper. Spray the paper with cooking spray.
Make the batter. Mix egg yolks and ½ cup of granulated sugar in a bowl of a stand mixer fitted with a whisk attachment on high speed until the foam is 3 times its original volume and falls in thick ribbons when the whisk is lifted. Transfer to a large mixing bowl, set aside.
Place egg whites in a clean bowl and beat them with a clean whisk attachment on medium speed until frothy. Gradually add the remaining ¼ cup of granulated sugar and continue to whip until medium peaks form when the whisk is lifted.
Fold the egg whites into the yolk mixture with a rubber spatula in 2 or 3 additions. Scatter all flour, vanilla, and salt over the batter and fold it just long enough to blend the flour evenly.
Prepare the gelatin. Pour ½ cup of water over the gelatin and let stay while the cake is baking.
Bake. Pour the batter into the prepared pan and spread gently with a spatula. Bake for 30-35 minutes until the top is golden brown and the edges star to pull away from the sides of the pan. Remove from the pan and place on a wire rack to cool.
Prepare the soufflé. Beat the butter on high speed until it starts whitening, slowly add the condensed milk.
Place soaked gelatin into a small sauce pan and add 125 g of sugar. Cook on low heat until it reaches 140⁰F/60⁰C.