Preheat the oven to 350 degrees. Spray a 6 cavity donut pan with non-stick spray.
In the bowl of a stand mixer fitted with the paddle, beat together the peanut butter, greek yogurt, and brown sugar until well combined.
Beat in the egg, milk, and vanilla.
In a small bowl, combine the flour, baking powder, and salt. Slowly incorporate into the wet mixture, mixing until just combined.
Spoon the batter into the donut pan and bake for 8 minutes or until the donuts spring back when lightly pressed.
Let donuts cool completely before frosting.
To make the frosting, beat together the butter, marshmallow creme, powdered sugar, milk, and salt until well combined.
Carefully spread onto the tops of the cooled muffins.
Melt extra peanut butter in a microwave safe bowl on low heat for about 30 seconds or until melted.
Drizzle the peanut butter over the frosted donuts. Serve immediately.