Preheat your oven to 350°F (175°C) to ensure even baking.
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar using an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
Add the peanut butter, eggs, and vanilla extract to the creamed mixture and mix on low speed until fully combined, scraping down the sides as needed.
In a separate bowl, whisk together the instant chocolate pudding mix, flour, baking soda, and baking powder.
Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.
Gently fold in the chocolate chips and mini marshmallows using a spatula, being careful not to overmix to keep the marshmallows intact.
Using a cookie scoop, drop rounded tablespoons of dough onto a parchment-lined baking sheet, spacing them a few inches apart.
Bake the cookies in the preheated oven for 10-12 minutes, until the edges are lightly golden but centers remain soft.
Remove from oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.