Start by preheating your oven to 350°F (175°C). This ensures your cookies bake evenly and achieve that perfect golden-brown color.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. This step is important as it evenly distributes the baking soda and salt, leading to a well-risen cookie.
In a large mixing bowl, combine the softened unsalted butter and peanut butter. Use a wooden spoon or electric mixer to cream them together until smooth. Gradually add in the granulated sugar and brown sugar, mixing until fluffy and well combined.
Beat in the egg and vanilla extract into the butter and sugar mixture. Mix until fully incorporated, ensuring the egg is well blended for a rich texture.
Slowly add the dry ingredients to the wet ingredients, mixing with a spatula until just combined. Be careful not to overmix; a few flour streaks are okay.
Gently fold in the rolled oats and Reeses Pieces into the cookie dough. This is where the fun happens! Make sure the candies are evenly distributed throughout the dough.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto a lined baking sheet, leaving enough space between each cookie for spreading.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden. The centers may look slightly underbaked, but they will continue to cook as they cool.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a cooling rack. This helps them set and makes them easier to handle.