Preheat your oven to 350°F (175°C) for even baking.
Cream together the room temperature unsalted butter, granulated sugar, and brown sugar in a large mixing bowl with an electric mixer until light and fluffy (2-3 minutes).
Add creamy peanut butter, eggs, and vanilla extract to the mixture, mixing until well combined.
In a separate bowl, whisk together all-purpose flour, baking soda, sea salt, and quick-cooking oats.
Gradually mix the dry ingredients into the wet ingredients on low speed until just combined.
Fold in the Reeses Pieces gently with a spatula until evenly distributed.
Scoop rounded tablespoons of dough onto prepared baking sheets, leaving space for spreading.
Optionally, sprinkle a tiny pinch of flaky sea salt on top of each cookie before baking.
Bake for 10-12 minutes until edges are lightly golden; centers may appear slightly underbaked.
Let the cookies cool on the baking sheet for about 5 minutes, then transfer to a wire rack to cool completely.