Preheat oven to 350 degrees. Line a baking sheet with parchment paper or a Silpat mat, set aside.
In the bowl of a stand mixer, beat together butter, sugars, vanilla and peppermint for 2-3 minutes. Add melted chocolate chips, mix until combined. Add egg, then egg yolk, mixing just until combined.
In a medium size mixing bowl, whisk together flour, cocoa powder, baking soda, and salt. With mixing speed on low, add dry ingredients. When no flour pockets remain, turn off and remove bowl from stand. Fold in M&Ms and peppermint baking chips. Scoop onto the prepared baking sheet.
Place in oven and bake for 10-12 minutes, cookies should be firm around the edges but soft in the center. Remove and allow to cool on pan for 2-3 minutes before transferring to a wire rack to cool completely.