Place the milk chocolate in a stainless steel bowl and set it over a pot of simmering water. Make sure the bottom of the bowl doesn’t touch the water. Melt the chocolate, stirring constantly with a rubber spatula. Before the chocolate is all melted, remove the bowl from the heat and stir it as it finishes melting – this prevents the chocolate from overheating. Let it cool at room temperature without allowing it to harden, about 15 minutes.
While the chocolate is cooling, place the peanut butter, sugar and salt in the bowl of a food processor. Process for 3 to 5 minutes, making sure the sugar and salt are all dissolved in the peanut butter. Add the cooled chocolate and process until everything is well mixed. Add the butter and continue to process until the butter is all melted, about 2 more minutes. At this point the mixture will be slightly warm.
Evenly divide the mixture between two plastic containers. Let cool to room temperature then cover with a tight lid and place it in the refrigerator for at least 6 hours, preferably overnight. (The filling can be kept in the refrigerator for up to 2 weeks).
Using a teaspoon, make little balls using the palm of your hands and place them on a flat baking sheet lined with parchment paper. As you work, the heat of your hands will slightly melt the filling and will stick in your hands. Clean your hands often and work with half a batch at a time, keeping the other half cold in the refrigerator. Chill the balls in the refrigerator to set, about 30 minutes. Remove them from the refrigerator about 3 to 5 minutes before dipping them in the melted chocolate to avoid a shock of temperatures.
Crumble the paçocas with your hands until it’s a coarse powder and place it in a large shallow dish.
Melt 2/3 of the bittersweet chocolate in a metal bowl set over a pot of simmering water. Make sure the bottom of the bowl doesn’t touch the water. Stir the chocolate with a spatula. As with the prior chocolate, remove the bowl from the heat before it is all melted and finish the melting by stirring it. Add the remaining 1/3 of chopped chocolate to the bowl (off the heat). Stir with a rubber spatula to melt the new chocolate into the already melted chocolate and leave it at room temperature for about 10 minutes (at this point the chocolate should be at 88 F).
Organize your working space with the tray of truffles on your left, the melted chocolate in the middle, and the crumbled paçoca on your right. Using a chocolate fork, dip each peanut truffle into the melted chocolate, covering the whole outside surface. Lift each truffle out of the chocolate and shake the fork gently up and down to let the excess chocolate drop off. Immediately roll each truffle in the crumbled paçoca until it’s fully covered. Let the chocolate fully set before removing truffles from the crumbled paçocas. Alternatively, you can just sprinkle some crumbled paçoca on the top of the truffle immediately after dipping and place on a baking sheet to set.
Place the truffles in a plastic container with a tight-fitting lid and keep them in a dry place at cool room temperature for up to 3 weeks.