Combine peanut butter, sugar, butter, and vanilla in mixer bowl. Stir until blended, mixture will be slightly dry.
Roll mixture into 1-inch balls and place them in the muffin tins or on prepared cookie sheets.
Stick a toothpick in the top of each peanut butter ball. You’ll use this “handle” later for dipping. Freeze the balls for about 30 minutes, until hard.
Pour semi-sweet and bittersweet chocolate chips into glass bowl. Add the butter. Microwave 60 seconds at 70% then gently stir. Microwave for 30 seconds at 70% and stir, repeating until the chocolate is almost but not completely melted. Do not overheat. You don’t want the chocolate to be completely melted in the microwave. If you’ve done it right, the last few stirs will mix in the remaining firm chips.
Dip the peanut butter balls into the chocolate, leaving a circle of peanut butter showing on top. They should look like buckeyes (see photo below). Place the dipped balls back on the cookie sheets.
Refrigerate finished buckeyes for at least 2 hours. Remove the toothpicks and store in tins between layers of wax paper.