Make the ganache: Heat the heavy cream in a saucepan until boiling. Pour the hot cream over the chocolate in a heat-safe bowl. Let it sit for 5 minutes, then stir until the chocolate melts and you have a smooth, shiny ganache.

Make the mascarpone cream: In a bowl, whisk together the chilled heavy cream, sugar, and vanilla extract until the cream is thick. Then, add the mascarpone cheese in 2-3 additions and mix on low speed until just combined. You can adjust the amount of mascarpone cheese depending on how thick and rich you want the cream to be.

Assemble the tiramisu: In a deep dish, mix the cocoa powder with the cooled coffee. Dip each ladyfinger in the coffee mixture for about 2 seconds per side. Arrange the soaked ladyfingers in a single layer in a 9x9 inch pan.

Layer the tiramisu: Spread half of the mascarpone cream mixture over the ladyfingers. Then, pour half of the ganache over the mascarpone cream. Repeat with another layer of soaked ladyfingers, mascarpone cream, and ganache.

Chill and serve: Finally, dust the top of the tiramisu with cocoa powder. Cover the tiramisu with plastic wrap and refrigerate for at least 4 hours, or ideally overnight, before serving.