Preheat your oven to 325°F and grease a donut pan with cooking spray. Use a paper towel to wipe away any excess.
In a large bowl, whisk together the flour, sugar, baking powder, and salt until well combined.
In another bowl, whisk together the milk, egg, vanilla extract, and vegetable oil. Slowly mix the wet ingredients into the dry ingredients until just combined. Gently fold in the chopped Reese’s peanut butter cups.
Transfer your batter into a piping bag or a ziploc bag with one corner snipped off. Pipe the batter into the donut pan, filling each cavity about two-thirds full.
Bake for 10 minutes or until a toothpick inserted in the center comes out clean. Let the donuts cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.
For the chocolate topping, combine the chocolate chips and 1 tablespoon of vegetable oil in a microwave-safe bowl. Microwave on 60% power, stirring every 30 seconds, until melted and smooth.
Carefully dip each cooled donut into the melted chocolate, allowing any excess to drip off. Place the dipped donuts on a wire rack to set for 3-4 minutes.
For the peanut butter drizzle, combine the creamy peanut butter and 1/2 teaspoon of vegetable oil in another microwave-safe bowl. Microwave on 60% power for 1-2 minutes or until melted.
Drizzle the melted peanut butter over your chocolate-covered donuts using a spoon.
Serve and enjoy these delightful Stuffed Peanut Butter Cup Donuts, or store them in an airtight container for later enjoyment!