Preheat oven to 350°F (177°C). Line 2 large baking sheets with parchment paper or silicone baking mats. (Always recommended for cookies.) Set aside.

Toss the flour, salt, and baking soda together in a large bowl. Set aside.

In a medium bowl, whisk the melted butter and brown sugar together until no brown sugar lumps remain. Whisk in the egg, then the peanut butter until combined. Finally, whisk in the vanilla. Pour the wet ingredients into the dry ingredients and mix together with a large spoon or silicone spatula. The dough will be very soft and slick. Fold in the oats and the M&Ms. Allow the cookie dough to sit for 15 minutes—the oats will absorb some moisture, making the dough easier to work with.

You can use the photos above as visuals for this step. Take 1 Tablespoon of dough and slightly flatten out on the prepared baking sheet. Stick a peanut butter cup on top, as shown above. Top the peanut butter cup with 1.5 Tablespoons of cookie dough and seal down the sides so that the peanut butter cup is securely stuffed inside. You can pick it up and roll into a smooth ball if needed. Slightly flatten down the top as best you can (it can’t be totally flat because of the peanut butter cup inside). Repeat with the rest of the dough and peanut butter cups.

Bake the cookies for 12-13 minutes. The cookies will look very soft. They will continue to bake on the cookie sheet. Remove from the oven and lightly press down on each cookie to slightly flatten (if they didn’t spread much in the oven). Allow to cool on the cookie sheet for 10 minutes before transferring to a wire rack to cool completely.