Over medium heat, melt white chocolate in double boiler. Once melted, set aside to cool until warm.
In a medium bowl, dissolve 1/2 tsp. of espresso in the sherry. Set aside.
To the chocolate, whisk in the mascarpone, espresso and wine mixture, vanilla and pinch of salt to combine. Add remaining 1/2 tsp. of espresso, mix to combine. Cover bowl and refrigerate overnight.
The following day, on a flat surface grate dark chocolate block and toss shavings with cocoa powder. Remove ganache from refrigerator, and scoop balls with a round spoon. One at a time roll into a ball using your hands, then roll the truffle in the shaved chocolate/cocoa until covered. Repeat.