Set aside the sweetened condensed milk and 200 g margarine. Dissolve the soda in the vinegar and add it as you beat the remaining ingredients together in the mixer.
Cover a medium baking pan with greaseproof paper and spread the mixture carefully over it. Preheat the oven to 180oC and bake. When it is ready, set aside to cool. Using a knife, cut the sides and crumble them. Place the 2 tins of sweetened condensed milk in boiling water and leave them for about 2 hours (make sure they are continually covered with water). (You can also find ready boiled sweetened condensed milk on the market.)
When the boiling is finished, let the tins cool slightly and beat the milk with the margarine in the mixer. With a length of thread, cut across the cake and spread a layer of cream. Cover the top of the cake with the remaining cream. Sprinkle the cake crumbs over the top and wrap in cellophane so the cake stays fresh.