Preheat oven to 350 degrees F (175 degrees C). Line an 8x8 baking dish with parchment and lightly coat with cooking spray.

Place peanut butter cups in a medium microwave-safe bowl and microwave in 30 second intervals, stirring in between, about 2 minutes total, until smooth. Let cool slightly.

Using an electric mixer or whisk, beat eggs until frothy . Slowing drizzle in chocolate while mixing until well incorporated. Pour batter into prepared pan.

Bake in the preheated oven until a toothpick inserted in center comes out clean, 22 to 26 minutes. Let cool completely before cutting.