Mix the peanut butter, powdered sugar, salt, and vanilla until smooth. Fold a piece of parchment paper in half. Transfer the peanut butter mixture to one half and fold the other side of the paper over so the peanut butter is in the middle of the two sheets. Smooth the peanut butter mixture to an even ½ inch thickness. Set this in the freezer for about 20 minutes. While that is chilling you can start with the chocolate.
Melt the chocolate to package instructions and carefully smooth it into each corner of the heart molds with a small rubber spatula. You want to have a solid and even coating on the hearts. To check coverage, carefully turn the mold around and look at it from each direction. Make sure no spots are missed. I like to have the silicone mold on a tray or plate while doing this as it helps with stabilizing the tray while the chocolate is setting up. You will have plenty of chocolate left over in your melting dish after this step, you will use this in the next steps. Leave the tray on the counter while the peanut butter filling chills.
Remove the filling mixture from the freezer and freehand cut, or use a cookie cutter to cut out hearts that will easily fit into the center of the chocolate hearts.
The chocolate should still be very liquidy and the peanut butter hearts should easily push right into the chocolate. Try to press them in just enough that they are in all the way but not so far that they would likely push through the chocolate top.
Pour the rest of the chocolate over the hearts and carefully smooth out so the chocolate evenly fills all of the space around and over the peanut butter filling. Aim for a flat bottom.
Set into the freezer to chill for about 20 minutes.
Remove the completely solid hearts from the mold. I like to use food-safe gloves to avoid fingerprints.
Set the hearts right onto the same cold/frozen tray that the silicone mold was on in the freezer.
Use the remaining melted chocolate to drizzle over the top of some or all of the hearts.
Optional: Top with nonpareils.
Store in an airtight container for up to a week.