First, gather all your ingredients on the counter. You’ll thank yourself later for making this step a habit. Preheat your oven to 350°F (175°C). Grease a round cake pan with butter and line the bottom with parchment paper. This will ensure your cake pops out easily when it's done.

Next, in a small saucepan, combine the butter, chocolate, milk, and a pinch of salt. Heat over low heat, stirring constantly until everything is melted and combined. Remove from heat and let it cool slightly. Why do I say this? If you pour hot chocolate into the egg yolks, they’ll scramble—no one wants scrambled yolk in their cake!

In a mixing bowl, whisk together the egg yolks and granulated sugar until the mixture lightens in color and thickens, about 4-5 minutes. It should look almost pale, like a sunrise over the horizon.

Now, slowly fold the slightly cooled chocolate mixture into the egg yolk mixture. Use a spatula to gently combine the two. Stir in the vanilla and espresso. This step brings depth to the flavor, enhancing the chocolate's richness.

In a separate bowl, whip the egg whites until they form stiff peaks. This is where the magic happens! The air you incorporate will give the cake its light, airy texture. You'll feel like a professional dancer as you whip those whites!

Gently fold the whipped egg whites into the chocolate mixture. Be careful here—not to deflate the fluffiness you just created. This step is crucial for achieving the cake's signature lightness.

Pour the batter into your prepared pan and smooth the top. Bake in your preheated oven for about 25-30 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. The cake should rise slightly but not too much—think of it as a gentle sigh.

Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes. Carefully invert the cake onto a wire rack to cool completely. The anticipation builds as the chocolate aroma fills the air.