Heat the oven to 325°F. After baking, let the parchment paper overhang to allow cake removal easy. Place egg yolks in a big bowl and egg whites in another clean, dry basin.
Add sugar to egg yolks and beat on medium-high until smooth and pale.
Pour in melted butter, vanilla, espresso powder, and salt while mixing on low. Beat 1–2 minutes until well combined.
Mix the flour and cocoa powder with a rubber spatula. Mix well, scraping sides as needed.
Next, slowly add 1/2 cup of milk to the chocolate mixture and beat until fully blended and consistent in color. Set aside.
In a separate dish, whip egg whites until firm peaks form. Avoid under- or over-whipping egg whites.
Gently incorporate egg whites into chocolate batter. Stir gently to avoid deflating egg whites. Do not mix until smooth or layers will not develop.