In the bowl of a stand mixer fitted with a paddle attachment, beat the peanut butter and butter on medium speed until smooth and creamy, about 2 minutes. With the mixer on low speed, gradually add the powdered sugar, vanilla, and salt. Beat until smooth, 1 to 2 minutes, scraping the sides and bottom of the bowl, as needed.

Using a tablespoon, divide the peanut butter mixture into about 28 portions, using your palms to quickly roll them into balls until smooth (chill the balls if they are getting too soft to work with). Place the balls on a parchment paper-lined baking sheet and freeze for 30 minutes. Reshape the balls once chilled, if needed.

Fill the bottom of a medium saucepan with about 1 inch of water and bring to a simmer over medium-high heat. Place the chopped chocolate in a dry, heat-proof glass or ceramic bowl that is slightly larger than the saucepan. The bowl should sit on top of the pan without its bottom touching the water.

Reduce the heat to medium-low. Place the bowl of chocolate over the saucepan of water. Heat the chocolate, stirring occasionally with a dry rubber spatula, until the chocolate is mostly smooth and melted, 3 to 5 minutes. Remove the bowl from the saucepan and continue stirring until the residual heat melts the remaining chocolate and the mixture is smooth (return the bowl to the saucepan, if needed). Pour the chocolate into a heat-safe mug.

Skewer 1 peanut butter ball with a toothpick. Dip the ball 3/4 of the way in the chocolate, leaving a small round of peanut butter showing. Allow excess chocolate to drip off. Return the ball to the parchment-lined baking sheet. Sprinkle the sides with jimmies, if desired. Remove the toothpick and carefully smooth over the hole. Repeat the dipping process with the remaining peanut butter balls, reserving the remaining melted chocolate for another use. Chill the buckeyes until the chocolate is set, about 30 minutes.