Line a baking sheet with parchment paper or a silicone baking mat. Set aside.
In a medium bowl, combine the peanut butter and softened butter. Mix with a spatula or hand mixer until smooth and uniform.
Gradually mix in the powdered sugar and cocoa powder until a smooth dough forms. If the dough feels too sticky to roll, add a little more powdered sugar, 1 tablespoon at a time, until it holds together. If it is dry or crumbly, mix in a teaspoon or two of milk or water until the mixture comes together.
Cover the bowl and refrigerate for 15-20 minutes, until the dough is firm enough to roll but not rock‑hard.
Scoop out about 1 tablespoon of dough at a time and roll it between your hands to form 1‑inch balls. Arrange the balls on the prepared baking sheet in a single layer. If the mixture softens as you work, chill the tray for 5-10 minutes before coating.
In a heatproof bowl set over a pan of barely simmering water, or in the microwave in 20‑second intervals, melt the semisweet chocolate with the vegetable oil, stirring frequently, until smooth and fluid. Do not overheat.
Working one at a time, dip each chilled ball into the melted chocolate, using a fork to turn and coat evenly. Let the excess drip back into the bowl, then gently slide the coated ball back onto the baking sheet.
Refrigerate until the chocolate is set and matte, about 15-20 minutes. Serve cold.