Beat the mascarpone, sugar and sour cream to a smooth cream mixture.
In another bowl, beat the liquid cream. If it's unsweetened, beat it with 4 tbsp sugar.
Combine the 2 creams and stir carefully.
Melt the white chocolate in a water bath and once it cools, add it along with the rum essence. Stir the cream again slowly and carefully until fully homogenized.
Dip the biscotti briefly in the milk and arrange them next to each other upright in a form with adjustable ring. Then cover the bottom of the form tightly with soaked biscotti as well.
Distribute 1/2 the cream on top of them. You can hit the form a couple times on the table to get all the air out. Arrange a 2nd layer of biscotti and smear them with the remaining cream.
Cover the form with transparent wrap and put in the fridge for about 10 hours.