In a small bowl, whisk the eggs, milk, and vanilla using a hand whisk or fork.
In a separate bowl, sift together the flour and baking soda, add sugar and salt, and mix well. Gradually whisk in the butter at low speed. Once a breadcrumbs like mixture is obtained, add half of the egg mixture and whisk well, then add the remaining egg mixture and whisk again until a homogeneous white mass is achieved.
Line the baking pan with parchment paper (cut to the size of the pan's diameter), grease it, and pour in the batter. Place in a preheated oven at 175°C and bake for 20-25 minutes. Check with a wooden skewer; if it comes out dry, the cake is ready. Cool the sponge cake in the pan for 5 minutes, then remove it and cool completely on a wire rack. Remove the parchment paper. Place the cooled sponge cake in a springform pan or line the sides with aluminum foil, which is needed when pouring the soufflé.