Step 1: Preheat the oven

Preheat your oven to 350°F (175°C). Line your muffin tin or mini cake tins with parchment paper or lightly grease them to prevent sticking. Set aside.

Step 2: Prepare the egg mixture

In a mixing bowl, separate the egg yolks from the egg whites. Place the egg whites in a clean, dry bowl, and the egg yolks in a separate bowl. Add a pinch of salt to the egg whites and beat with an electric mixer on medium speed until soft peaks form. Add the granulated sugar gradually and continue to beat until stiff peaks form. This process will help give the sponge cake its airy texture.

Step 3: Combine the egg yolks and dry ingredients

In another bowl, whisk the egg yolks and vanilla extract. In a separate bowl, sift the all-purpose flour to remove any lumps. Gently fold the sifted flour into the egg yolk mixture until well combined.

Step 4: Fold in the egg whites

Carefully fold the beaten egg whites into the egg yolk and flour mixture using a rubber spatula. Be gentle to avoid deflating the batter, as the airy texture is crucial for a light sponge cake.

Step 5: Add butter and milk

Melt the unsalted butter and warm the milk. Drizzle the melted butter and warm milk into the batter, folding them in gently. This adds richness to the cake while maintaining its light texture.

Step 6: Bake the cakes

Spoon the batter into the prepared muffin tin or mini cake tins, filling each cup about 2/3 full. Bake for 12-15 minutes or until the cakes are golden brown and a toothpick inserted into the center comes out clean. Allow the cakes to cool slightly before removing them from the tin.