Preheat the oven to 350 F. Line a 12-cup muffin pan with paper liners. In a medium bowl, whisk together flour, salt, and baking soda.
In a large bowl, combine butter with peanut butter and both sugars until fluffy. Add in egg and vanilla extract, and mix until smooth. Add in the flour mixture, mix until it is fully combined. Fold in Reese's pieces.
Evenly divide the cookie dough between the lined muffin cups. Bake for 15-20 minutes.