In a medium bowl, combine 1/2 cup butter and cream cheese. Beat with an electric mixer on medium speed until mixture is combined. Add flour. Beat on low speed just until combined. Cover and chill dough for one hour.
Shape dough into 24 balls. Press each ball evenly into the bottoms and up the sides of 24 ungreased 1-3/4-inch mini muffin cups.
In a small saucepan, combine 1/2 cup chocolate pieces and 2 tablespoons butter; heat and stir over low heat until melted. Remove from heat. Whisk in sugar, egg, and vanilla. Spoon chocolate mixture evenly into the pastry-lined cups.
Bake in the 325F oven for 20 to 25 minutes or until filling is puffed and pastry is golden brown. Quickly top tassies with pecans, the 1/3 cup chocolate chips, and the marshmallows. Bake 1 to 2 minutes more or until marshmallows are softened. Remove the tassies from the muffin tray and cool on a wire rack.