If using a muffin pan, insert muffin liners and set the pan aside. If using a silicone rose mold (see note), you don't need to prep the mold at all.
Melt the chocolate using a double-boiler method (for more specifics, see notes). Once melted, spoon an even layer of melted chocolate into the bottom of each liner/mold. Try to keep the layer thin, but make sure it coats the surface evenly.
Set the pan aside to allow the chocolate to set, or place it in the freezer for 5-10 minutes to speed up the process.
Once the chocolate is set, dollop about 1/2-1 Tablespoon of peanut butter into the center of each cup, and use an offset spatula or the back of a spoon to smooth it into an even layer. Try to avoid getting peanut butter on the sides of the liner/mold.
Return the pan/mold back to the freezer. In the meantime, remelt the remaining chocolate if need be.
Once the peanut butter has hardened, remove the pan/mold from the freezer, and top each cup with an even layer of melted chocolate – enough to cover the peanut butter and leave a smooth top.