Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Combine peanut butter, brown sugar, and granulated sugar in a mixing bowl. Cream until light and fluffy, about 2-3 minutes.

Add the egg and vanilla extract, and beat until fully incorporated.

Mix in baking soda and salt gently until just combined.

Fold in M&M's, leaving some aside for later.

Drop rounded balls of dough onto the prepared baking sheet, spacing them apart. Flatten slightly and add reserved M&M's on top.

Bake for 8-10 minutes until edges are set but centers look slightly underbaked.

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