In a large mixing bowl, combine the softened butter, peanut butter, brown sugar, and granulated sugar. Beat on medium speed for 2 to 3 minutes until light, fluffy, and smooth.

Add the egg and vanilla extract. Beat until fully incorporated, scraping down the sides of the bowl as needed.

In a separate medium bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet mixture. Mix on low speed just until a soft dough forms.

Fold in the peanut butter chips and Reese's Pieces until evenly distributed.

Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes.

Preheat the oven to 350°F and line two large baking sheets with parchment paper or silicone baking mats.

Scoop the chilled dough into 2-tablespoon-sized balls. Roll them gently and place them about 2 inches apart on the prepared baking sheets.

Bake for 9 to 11 minutes, until the edges are lightly golden and the centers still look slightly soft.

Let the cookies cool on the baking sheets for 5 minutes.

Press additional Reese's Pieces onto the warm cookies and sprinkle with flaky sea salt, if desired.

Transfer the cookies to a wire rack to cool completely.