Beat sour cream with two tablespoons of rum and lemon zest, gradually add the caramel mass and beat until cream.
From one lemon, grate the zest in a cup. Pour 200 g of sugar there and squeeze the juice from 4 lemons. Mix everything with a spoon. In another cup, beat the eggs with a fork, not until foam.
Combine and mix both mixtures, but without fanaticism. Leave for 30 minutes to give the zest its flavor. Filter through a sieve into a saucepan in which we will prepare the cream. This should be done in order to get rid of lemon seeds, under-mixed proteins, which, when heated, can curl up, and zest.
We put the saucepan over medium heat and begin, without stopping, stir the cream. As soon as the cream begins to thicken, add butter to it and mix again until thickened. At the beginning, the cream will remain liquid for some time, and then it will rapidly begin to thicken. To chill.