Beat the softened butter with the cream cheese, then blend in the vanilla once the mixture becomes fluffy. Look for an evenly combined mixture without visible cream cheese lumps.
Stir the salt through the flour in a separate bowl, then work this mixture into the butter mixture in four additions, incorporating each portion before adding the next. The flour should disappear into a cohesive, soft dough.
Enclose the dough in plastic wrap and refrigerate for 1 hour. This chill firms the fat so the dough becomes easier to handle.
Set the oven to 400°F and allow it to preheat. The oven should reach the baking temperature before the cookies go in.
On a lightly floured work surface, roll the dough to approximately ⅛-inch thickness and cut it into 2-inch squares. A consistent thickness helps the cookies bake evenly.
Portion a little more than a ½ teaspoon of fruit pastry filling onto each square’s center and draw it into a diagonal strip. Keep the two corners that will fold together free of fruit.
Bring the two clean, opposite corners over the filling, dampen their contact area with a little water, press the overlap firmly together, and arrange the cookies about an inch apart on a baking sheet. The overlapping layers should touch dough to dough, with no filling between them.
Bake for 12 to 15 minutes, transfer the cookies to wire racks to cool completely, and dust with powdered sugar before eating. Look for set edges and lightly golden bottoms rather than deeply browned tops.