MAKE THE FILLING: Place the peanut butter and butter in a large bowl and mix well.
FORM THE TRUFFLES: Pour the mixture into a 2-inch-deep baking pan, spread evenly, and place in the freezer for 30 minutes or until set (it should have the consistency of fudge). Using a melon baller or a small spoon, form rounds and place them on a baking sheet lined with parchment or wax paper. Let the truffles harden in the freezer for about 15 minutes. After removing from the freezer, roll truffles between your hands into marble-size spheres, squeezing slightly (try to do this quickly, otherwise they'll become too soft). You can now dust the truffles with cocoa and serve them as is, but they'll hold their shape better if you coat them with chocolate first.
MAKE THE COATING: Let the truffles rest in the freezer while you make the chocolate glaze. Place the remaining chocolate pieces in a large bowl over a saucepan of simmering water and stir occasionally, until the chocolate is completely melted. Remove from heat and let cool at room temperature, stirring occasionally, until the chocolate starts to set at the edge of the bowl. Drop the truffles into the melted chocolate and retrieve them with a fork, allowing any excess chocolate to drip off. Garnish immediately or leave the truffles plain and proceed to step 5.
GARNISH: Set the freshly coated truffles on a plate and sift the cocoa powder over them. Turn the truffles and sift again to cover completely.
STORAGE: Place the truffles on the lined baking sheet and allow them to set in the refrigerator for 5 minutes. Truffles will keep for about 2 weeks, chilled or at room temperature, when stored in a tightly sealed container.