Preheat oven to 350F and line baking sheets with parchment paper.
In a large bowl, or a bowl of a stand mixer, add the softened butter, white sugar and brown sugar. Beat with the paddle attachment for 2-3 minutes, until creamed together. It should be light in color and fluffy.
Add in the egg and vanilla and beat again until incorporated.
In a separate large bowl, mix together the oats, GF flour, peanut butter powder, baking soda and salt.
Add the dry ingredients to the wet ingredients and beat until just combined and a bit of flour remains visible. If the dough is getting to thick to use the mixer, switch to a spatula and mix it manually.
Then, add the chocolate chips & M&Ms and fold into the dough until incorporated and no more flour is visible.
Scoop out 2oz of dough and place onto cookie sheet, leaving about 2 inches between each dough ball as they will spread. Press a few extra M&Ms and chocolate chips into the top of the dough balls.
Bake for 13 minutes, or until the edges are just lightly golden. Immediately upon removing from the oven, top with a pinch of flaky sea salt if desired. Let them sit on the baking sheet for 5 minutes before transferring to a wire rack.
Once fully cooled, store cookies in an airtight container at room temperature. They should stay good & soft for 3-5 days!