a basic ganache is an emulsion of 1 part chocolate to 1 part cream. (used as maybe a chocolate glaze) while a firmer ganache of 2 parts chocolate to 1 part cream works well for chocolates. (there are really a lot more complicated ratios for chocolates but lets leave it at that for now..) but for the crostata something a little more soft but also a little more firm works well. so what was used was:
3 parts - dark chocolate (50% ish is fine)
2 parts - cream
and if you want to add a little sheen, add a little butter at the end around 1/2 part.
chop chocolate very fine. if you find thin chocolate pellets they work well also. heat cream to boil. (if you want to infuse a flavour add as cream is heating or leave to infuse in the cream overnight before boiling.) drop the hot cream onto the very finely chopped chocolate. it is best to use quite a narrow container so as not to lose to much heat. i generally use the beakers of emersion stick blenders as they are also useful in case the ganache breaks. after pouring on the hot cream (filter with sieve if having infused spices or zest) wait 30 seconds for the the heat to melt the chocolate. if you start stirring immediately you may lose to much heat and the chocolate won't melt. if the ganache breaks, which at first means it loses its silky smooth sheen and at worst begins to clump and become grainy, add a little cream (cold is fine) and use the stick blender to reincorporate the emulsion. you will immediately see the silky smooth emulsion returning. works 'almost' every time.
pour finished ganache. let sit at room temperature for several hours and then let chill in the fridge to allow crystallisation to finish.
although the procedure for ganache is straight forward, its nature is quite complex. the best discussions i have seen are on joepastry.com.