Set aside 6 tumblers or low-ball glasses for serving the tiramisu.
Separate the egg whites and yolks into two separate medium-sized bowls. With an electric hand mixer, beat the egg whites until stiff peaks form. Set aside.
Add the sugar to the bowl with the egg yolks and beat with the mixer until pale yellow in color, about 5 minutes. Add the mascarpone and beat again until combined. Using a spatula, gently fold the beaten egg whites into the mascarpone mixture until evenly combined.
To assemble the tiramisu: quickly dip the first biscuit completely in the cooled espresso and then place into one of the serving glasses, trimming it as necessary so it can lay flat in the base of the glass. Repeat until the base is completely covered in biscuits, trimming them as necessary to fit. Top the biscuit base with a spoonful of the mascarpone mixture and then top that with a thin sprinkling of cocoa powder. Continue layering the dipped biscuits, mascarpone, and cocoa powder until you reach the top of the glass, ending with the mascarpone. Repeat with the remaining serving glasses. Cover each glass with plastic wrap, transfer to the refrigerator and chill overnight.
When ready to serve, remove the serving glasses from the refrigerator and sprinkle each with cocoa powder to garnish. Serve immediately.
There are raw eggs in this recipe. Be sure to use the freshest eggs you can find.