In a large mixing bowl, combine the creamy peanut butter, very soft unsalted butter, vanilla extract, and salt. Use an electric mixer to beat the mixture until it's fluffy and well combined.
Gradually add the sifted confectioners' sugar to the peanut butter mixture, mixing on low speed until incorporated. Once combined, add the milk a little at a time to achieve a smooth, scoopable consistency.
Gently fold in the finely chopped HEATH toffee bars and honey roasted peanuts, if using.
Using a cookie scoop or a tablespoon, portion out the peanut butter mixture and roll it into balls about 1 inch in diameter. Place the balls on a parchment-lined baking sheet. Place the baking sheet in the freezer for about 30 minutes to firm up.
While the peanut butter balls are chilling, melt the finely chopped dark chocolate in a double boiler or microwave. If using espresso powder, add it to the chocolate as it melts.
Once the peanut butter balls are firm, remove them from the freezer. Dip each ball into the melted chocolate, leaving the top exposed. Use a fork to lift the dipped ball and let excess chocolate drip off before placing it back onto the parchment-lined sheet.
After all the buckeyes have been dipped, place the baking sheet back in the refrigerator for about 30 minutes to allow the chocolate coating to set completely.
