In a large bowl, whip the heavy cream with sugar until stiff peaks form.
In another bowl, combine the mascarpone cheese with cocoa powder until smooth, then gently fold in the whipped cream. Set aside.
Heat milk in a saucepan until warm, then stir in chocolate chips, instant coffee (if using), and vanilla extract until smooth.
Dip each ladyfinger quickly into the hot chocolate mixture (do not soak).
Layer dipped ladyfingers in a square or rectangular dish.
Spread half the mascarpone mixture on top. Repeat the layers.
Refrigerate for at least 4 hours or overnight.
Top with chocolate shavings before serving.