In a stand mixer with the paddle attached, beat the mascarpone (or cream cheese, if you're substituting) over medium speed until light and fluffy.
With the motor running, add the sugar and salt slowly, and continue mixing until incorporated.
Turn the speed to low and add the cooled custard, a quarter cup at a time, until fully blended.
Assemble the tiramisu (see notes after the recipe)
Split the cooled sponge cake horizontally into two.
Stir the spiced rum into the cooled coffee.
Using a pastry brush, dab half of coffee-rum mixture on the top of bottom half of the sponge cake.
Spread half of the cheese mixture on the coffee-soaked sponge cake.
Take the top half of the sponge cake and press lightly on the cheese mixture.
Dab the remaining half of the coffee-rum mixture on the top half of the sponge cake.
Spread the remaining half of the cheese mixture on the top half of the sponge cake.
Scatter shaved dark chocolate on top of the cake.
Chill for several hours before slicing and serving the tiramisu.
I developed this recipe years ago when I was supplying cheesecakes to a bistro in the suburb. I didn't want to take risks with customers complaining of salmonella infection so I experimented until I was satisfied that the tiramisu with cooked egg yolks was just as delicious as traditional tiramisu with raw eggs.To make sure that your tiramisu will not fall apart during assembly, assemble it in a ten-inch cheesecake pan — springform or with a removable bottom. Drop the first layer of sponge cake into the pan and proceed as above but do not add the shaved chocolate on top yet.Wrap the pan in cling film, chill the assemble tiramisu in the fridge for about eight hours, unwrap and loosen from the pan. Transfer the cake to a plate (or a cake stand, or whatever you're using to serve it) then scatter the shaved dark chocolate on top.