Pour the cream into the bottom of a mixing bowl. Using either a hand or stand mixer, beat the cream on medium-high. With the mixer running, slowly pour the sugar and add the vanilla. Beat until stiff peaks form.
Add the room-temperature mascarpone to the whipped cream and fold in with a spatula until fully incorporated.
Mix the coffee and Kahlua (if using) in the bottom of a wide, shallow bowl. Quickly dip the lady fingers in the mixture (about 1 second on each side) and place on the bottom of an 8x8 (or roughly similar) pan. Use half of the ladyfingers to line the bottom of the pan.
Spread half the whipped cream mixture on top of the first layer of lady fingers.
Dip and layer the rest of the ladyfingers then spread the remaining whipped cream mixture on top. Dust the Tiramisu in cocoa powder, cover, and store for at least 4 hours before serving.