Preheat oven to 350F. Spray a 9-inch glass pie plate with cooking spray.

In a microwave-safe bowl, melt the butter on high for about 1 minute.

Let the melted butter cool slightly, then add the egg, brown sugar, and vanilla. Whisk until smooth.

Add the peanut butter and stir until combined.

Stir in the flour and salt until just combined. Do not overmix.

Fold in the sprinkles and Reese's Pieces.

Pour the batter into the prepared pie plate. Evenly space the peanut butter cups on top and gently push them down to be flush with the dough.

Sprinkle extra Reese’s Pieces on top for decoration.

Bake for 26 to 29 minutes until center is set and a toothpick inserted comes out clean or with moist crumbs.

Let the pie cool on a wire rack for at least 30 minutes before slicing and serving.