In a medium bowl, stir together the butter and cream cheese. Mix in the flour until well blended. Cover and refrigerate for 1 hour.
While the dough chills, make the filling. In a medium bowl, whisk together the egg, brown sugar, melted butter and vanilla. Stir in the cranberries and walnuts.
Preheat the oven to 325 degrees F (160 degrees C). Roll the dough into 1 inch balls. Press the balls into the cups of mini muffin pans so that the dough covers the bottom and sides. Fill each one with a generous teaspoon of the filling.
Bake for 25 to 30 minutes in the preheated oven, until the edges of the tarts are lightly browned.