In a large mixing bowl, using an electric mixer, beat butter and sugar until fluffy, add egg and beat until combined and light and fluffy. Mix in the pumpkin puree and vanilla extract.
Add flour and all dry ingredients. Mix on low speed until combined.
In a small bowl, toss roughly chopped peanut butter cup pieces in 1 Tbsp of flour until lightly coated. When the pieces have a flour coat, they won't all sink to the bottom of the batter while baking.
Save approx. 1/3 of the peanut butter cup pieces for topping the loaf before baking. Fold 2/3 of the pieces into the pumpkin batter.
Pour pumpkin batter into a greased loaf pan and place the saved Reese's pieces on top of the loaf however you think looks appealing. They will move a bit during baking, fyi.
Bake at 350 F for 60-70 mins until fully baked through - toothpick comes out clean from the middle of the loaf. Cool completely before slicing. Enjoy!!