In a bowl or the bowl of a stand mixer, beat the heavy cream on medium speed until stiff peaks form, about 5 minutes. Keep the whipped cream in the refrigerator until needed.
In the bowl of the mixer or another bowl, beat the mascarpone cheese, sugar, and vanilla extract until creamy, about 3 minutes.
Gently fold 1/3 of the whipped cream into the mascarpone mixture using a spatula, incorporating with circular movements to maintain the airiness of the whipped cream. Repeat with another 1/3 of the whipped cream, and after it is incorporated, fold in the remaining whipped cream.
In a bowl, mix the hot instant coffee with the coffee liqueur. Dip the savoiardi in the coffee mixture briefly, ensuring they are soaked but not soggy.
Arrange a layer of soaked savoiardi on the bottom of a rectangular pan (20x24cm). Spread half of the mascarpone cream mixture over the savoiardi layer. Add another layer of soaked savoiardi on top of the cream. Spread the remaining mascarpone cream evenly over the second layer of savoiardi.
Cover the tiramisu with plastic wrap and refrigerate for at least 2 hours, or preferably overnight. Before serving, dust the top with cocoa powder.