Beat soft butter for about 5 minutes to an airy, creamy mass.
Add sweetened condensed milk in portions and whip it up briefly each time to form a homogeneous cream.
Set the butter cream aside at room temperature.
Place egg whites in a large mixing bowl.
In a small, heavy-bottomed saucepan, combine sugar, agar agar, and citric acid.
Add water and stir.
Bring the sugar syrup to a boil over medium heat, stirring constantly so that the sugar dissolves before it boils, and simmer for 3 minutes.
At the same time, beat the egg whites in the mixing bowl until fluffy.
Immediately after boiling, slowly add the sugar syrup to the beaten egg whites in a thin stream, without stopping beating.
After you have introduced the sugar syrup into the egg whites, continue beating the mixture until stiff for about 2 - 3 minutes.
Now add portions of the butter cream that you previously set aside to the beaten egg whites and whip it into a homogeneous mass, which becomes a little runny.
Spread the soufflé quickly over the cake base in the springform tin and carefully place the second cake base on top, lemon side down.
Chill the cake in the springform tin for about 1-2 hours until the soufflé has set.