Place the egg yolk and sugar in the heatproof bowl, and whisk together until they become pale yellow. Set a heatproof bowl on top of a saucepan of simmering water, making sure the water doesn't touch the bottom of the bowl. Cook on low heat, stirring and whisking constantly, until the mixture thickens and forms a ribbon trail on the surface when the whisk is lifted (5-7 minutes). Remove from the heat and let cool slightly.
Whisk in vanilla extract. Beat in the mascarpone until smooth. In a separate bowl, beat the whipping cream until stiff peaks form. Gently fold into the cheese mixture.
The strawberries:
With a paring knife, cut off the strawberry stems. Cut around the inside of the strawberry, to create a well for the filling. Cut a thin slice from the bottom of each berry and stand upright on the cut side.
Add the filling to a piping bag and pipe into each strawberry, adding a little extra to the top. Keep chilled until ready to serve.
Right before serving, garnish each strawberry with a slice of ladyfinger and dust the tops with cocoa powder. Enjoy!
Notes
Don’t make more than 3 hours in advance, or your strawberries will get soggy.