In a bowl, whisk together the milk, eggs, salt, and vanilla extract just until smooth and combined.
In a separate bowl, mix the flour with the cocoa powder and vanilla sugar. Gradually fold this dry mixture into the milk-egg mixture, stirring constantly to avoid lumps.
Pour in the sparkling water and stir again until you get a smooth, thin batter. Let it rest at room temperature for about 15 minutes.
Heat a crepe pan and brush it lightly with oil. Pour about half a ladle of batter per crepe, swirl the pan to spread it evenly, and cook on both sides. Repeat with the remaining batter.
Let the crepes cool completely before filling.
Tiramisu Cream:
Combine the mascarpone, cold heavy cream, powdered sugar, and vanilla extract in a bowl. Beat on medium speed until you get a thick, smooth, fluffy cream. Keep chilled until ready to use.
Brew the coffee, let it cool completely, then pour it into a shallow bowl for dipping the ladyfingers.
Filling and Rolling the Crepes:
Lay one crepe flat on a work surface. Spread a thin, even layer of mascarpone cream over the entire surface.
Quickly dip a ladyfinger into the cold coffee (just a brief dunk so it moistens the outside without turning soggy) and place it at one end of the crepe.
Roll the crepe tightly around the ladyfinger. Repeat with the remaining crepes, cream, and ladyfingers.
Arrange the filled crepes side by side on a serving platter. Dust generously with sifted cocoa powder.
Refrigerate the platter for a few hours (ideally 3–4 hours) so the flavors meld and the cream firms up.
When serving, you can cut each crepe in half on the diagonal for a more attractive presentation.
Notes
Use very cold heavy cream for the fastest, most stable whip.
Let the crepe batter rest a full 15 minutes for a smoother texture.
Dip ladyfingers quickly — over-soaking makes them fall apart.
Chilling overnight gives the best flavor and cleanest slices.
A nonstick crepe pan makes thin, even crepes much easier to achieve.
Dust with cocoa just before serving to keep the top looking fresh.