Preheat the oven to 350°F (175°C). Line two large baking sheets with parchment paper.

In the bowl of your stand mixer fitted with a paddle attachment or large bowl with a hand mixer, beat peanut butter, butter, brown sugar, and granulated sugar on high speed until smooth and creamy.

Add the egg and egg yolk, then mix in the vanilla extract.

In another small bowl, whisk together the baking soda, salt, oats, and flour.

With the mixer on low speed gradually add the dry ingredients to the wet mixture, mixing until just combined.Do not over mix.

Fold in the m&ms, chocolate chips, and optional white chocolate chips and sprinkles.

Using a large cookie scoop (about 2 tablespoons), drop dough balls onto the prepared baking sheets, spacing them 2 inches apart.Optionally, top each cookie with more m&ms and chocolate chips.

Bake for 10–12 minutes, until the edges are golden. Centers may appear slightly underdone; they’ll set as they cool.

Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.