1. In a large mixing bowl, cream together the butter, sugar, and salt. Scrape the sides and bottom of the bowl, and beat in the vanilla and egg.
2. Add the flour and mix until the dough comes together; it will be soft.
3. Transfer the dough to a large plastic bag, flatten it, and refrigerate for at least 1 hour.
4. Preheat the oven to 425°F. Generously grease the heart molds with soft butter.
5. Working with 1/4 of the dough at a time, roll it 1/8" thick on a floured surface or piece of parchment paper. Flour the top of the dough and and place plastic wrap over it before rolling to keep it from sticking.
6. Cut out 3 1/2" hearts, lift them off the work surface with an offset spatula, and place into the bottom of the buttered molds. (Scrapes can be gathered and re-rolled.)
7. Put 2 teaspoons of pastry cream (a level teaspoon scoop) into each lined mold, cut a 3" heart from the pastry, and place it on top.
8. Using a table knife or offset spatula, scrape off any extra dough to seal the edges. Place the filled molds on a baking sheet, and bake for 15 to 17 minutes, until a deep golden brown.
9. Remove from the oven and cool on the baking sheet for 10 minutes before gently tipping the filled hearts out of the molds.
10. Serve slightly warm, dusted with confectioners' sugar. Hearts can be refrigerated for up to 3 days; reheat briefly before serving.