Place a heatproof bowl over a pot of gently simmering water (the bowl should not touch the water). Crack 3 eggs and separate the yolks from the whites into different bowls. Add the 3 egg yolks to the heatproof bowl. Beat the egg yolks with a whisk for about 3 minutes, stirring constantly, until they turn thick and pale yellow. This helps incorporate air and gives the mousse its light texture. Checkpoint: The egg yolks should look pale and ribbon-like when you lift the whisk.

Gradually add 1 cup sugar to the egg yolks, whisking constantly. This takes about 2–3 minutes. Pour slowly and beat well between additions to prevent lumps. Checkpoint: The mixture should be smooth and pale.

Continue beating the egg yolk mixture over the simmering water for about 5 minutes, stirring constantly. The mixture will double in volume and become very thick and custardy. You can test doneness by checking that the mixture reaches about 160°F with an instant-read thermometer (this makes it safe to eat). If you don't have a thermometer, beat for the full 5 minutes and look for a thick ribbon that slowly falls from the whisk. Remove the bowl from the water and set aside. Checkpoint: The mixture should be warm, thick, and pale enough to see a trail when you lift the whisk.

In a small bowl, whisk together 1/2 cup cocoa powder, 3/4 cup melted butter, 1/4 cup milk, and 1 1/2 tablespoons instant coffee granules. Stir until smooth and well combined, about 1 minute. This creates your chocolate base. Checkpoint: There should be no lumps of cocoa powder visible.

Fold the cocoa mixture into the warm egg yolk mixture using a rubber spatula. Use a gentle folding motion (cut down the center, sweep across the bottom, and fold up and over), rotating the bowl as you go. Fold for about 1–2 minutes until the cocoa is evenly combined and the mixture is uniform in color. Checkpoint: The mousse mixture should be deep chocolate brown with no streaks.

In a clean bowl, add the 3 reserved egg whites and 1/4 teaspoon cream of tartar. Beat with an electric mixer on medium-high speed for about 3–4 minutes until stiff peaks form. When you lift the beaters, the peaks should stand straight up without drooping. This takes time—don't rush it. Checkpoint: The egg whites should look glossy, smooth, and form sharp peaks.

In another clean bowl, pour 1 cup heavy whipping cream. Beat with the electric mixer on medium-high speed for about 2–3 minutes until stiff peaks form. Stop before it becomes grainy or turns into butter. Checkpoint: The whipped cream should form stiff, smooth peaks that hold their shape.

Using a rubber spatula, gently fold the stiff egg whites into the chocolate mixture using a folding motion. Work slowly, folding for about 1–2 minutes. You'll still see some white streaks. Checkpoint: Some white streaks are okay; stop folding when they're mostly combined.

Fold the whipped cream into the chocolate mixture using the same gentle folding technique. Fold for about 1–2 minutes until everything is evenly combined and smooth. Checkpoint: The mousse should be light, fluffy, and uniform in color with no visible streaks.

Pour the mousse into a serving dish or divide among 8 serving glasses or bowls. Use a spatula to gently scrape all of the mixture into the dish. The mousse should look light and fluffy. Chill in the refrigerator for at least 4–6 hours, or until set and cold. Overnight chilling is even better. Checkpoint: The mousse should be cold and firm enough to hold its shape when spooned.

Just before serving, top each portion with a dollop of whipped cream and a few chocolate curls. Serve cold. Each serving stays fresh in the refrigerator for up to 3 days.