Make dough:
Cut butter into bits and in a bowl with a pastry blender or in a food processor blend or pulse together with flour and salt until mixture resembles coarse meal. Add 2 tablespoons ice water and toss with a fork or pulse until incorporated. Add enough remaining ice water, 1 tablespoon at a time, tossing with fork or pulsing to incorporate, to form a dough.

Sautee shallot in butter. Add mushrooms, salt/pepper, cook 5 min. Add cream, boil. Cook until mixture reduces. Add 1 T truffle paste, cool. Roll out a square of puff pastry, brush with beaten egg. Cut out squares and place a spoonful of filling on ½ of them. Cut slits in the unfilled squares. Top filled with unfilled squares, and seal edges. Bake for 15 mins at 400.