Combine peanut butter, butter, vanilla and salt in large a bowl. Beat with an electric mixer on low speed until blended. Add powdered sugar, beating until perfectly combined and a firm, smooth dough forms. Note, you don't need an electric mixer for this; just use a wooden spoon and some stirring power, eventually giving up the spoon and simply kneading the mixture together with your hands until that smooth dough forms. You'll need it to be able to stick together in a ball shape with no crumbly bits.
Using a 1-tablespoon measure, shape the peanut butter mixture into balls and place onto a parchment/baking paper-lined tray. You should get around 2 dozen. Refrigerate for 30 minutes.
Meanwhile, melt the dark chocolate using your preferred method (microwave or double-boiler; I use the double boiler method). Transfer the melted chocolate to a tall, thin cup or glass.
Use a toothpick to stick into each of the peanut butter truffles and pick them up, one by one, and while holding the toothpick, dip the truffle into the melted chocolate, almost to the top, but not quite (refer to above photo).
If you find your truffle falling off the toothpick, use a fork, or extra toothpicks, to help hold the truffle on while scraping off the excess chocolate. It shouldn't fall off, though, if you work quickly. If your kitchen is warm, just take a few peanut butter truffles out of the refrigerator at a time to work with (the chilled truffles hold onto the toothpick better than when they're warm).
Place the dipped truffles back on the lined tray. When finished dipping, remove the toothpicks and very gently smooth over the toothpick-hole with your finger, or the back of a spoon.
Refrigerate the buckeyes for at least 30 minutes. Then, serve, or pack into your gift boxes!