Start by making the mascarpone cream. In a mixing bowl, whisk double cream with three tablespoons of powdered sugar and vanilla.
Whip the cream until soft peaks are formed.
Gently fold in the mascarpone and coffee liqueur (if using any) to the cream until well combined, then cover and refrigerate.
In a separate bowl, mix Nutella with some milk and set aside.
Mix warm coffee with the remaining tablespoon of powdered sugar in a small deep bowl.
Start assembling the tiramisu in a square pan.
Dip the ladyfingers in the warm coffee mixture and set it in the glass dish to form your base layer.
Evenly spread 1/3 of the Nutella mixture.
Now spread out 1/3 of the mascarpone mixture over the Nutella mixture.
Repeat the previous three steps.
By the end, you should have three layers of each tiramisu component.
Cover the dish with plastic wrap and refrigerate it for at least four hours, or overnight.
Dust the top of your Nutella tiramisu with cocoa powder.
Garnish with mint leaves and serve chilled.