1-First Step: Preheat your oven to 350°F. Grease a 9-inch round cake pan with butter and line the bottom with parchment paper. This prevents sticking and ensures easy release. Have all ingredients at room temperature for better whipping cold eggs reduce volume.

2-Second Step: Combine 4 large eggs and 2/3 cup sugar in a heatproof bowl. Place over a pot of simmering water (double boiler method) and whisk constantly for 4-5 minutes until warm (about 110°F) and tripled in volume. This sabayon technique stabilizes the mixture. Remove from heat immediately to avoid cooking the eggs.

3-Third Step: Beat with an electric mixer on high speed for 8-10 minutes until pale, thick, and reaches ribbon stage batter falls in a ribbon that holds for 5 seconds. Scrape sides often. This incorporates air crucial for the sponge cake rise.

4-Fourth Step: Sift 3/4 cup cake flour and 1/4 tsp salt together three times for lightness. Fold into the egg mixture in three additions using a rubber spatula. Cut down the center and sweep up gently 20 strokes max per addition. Avoid stirring to keep air bubbles intact.

5-Fifth Step: Warm 2 tbsp melted butter slightly. Fold in quickly by taking a bit of batter into the butter first, then incorporating back into the main bowl. Pour into prepared pan immediately; batter loses structure fast.

6-Sixth Step: Bake on middle rack for 25-30 minutes. Top turns golden, and cake springs back when touched lightly. Do not open oven door early to prevent collapse. Invert onto a wire rack right away, propping on a bottle neck to cool upside down for 1 hour this sets the structure.

7-Final Step: Run a knife around edges, remove from pan, and peel parchment. Cool fully before slicing or filling. Yields one 9-inch layer, perfect for cake batter experiments. Store wrapped if not using same day. Total time: 1 hour active plus cooling.