Preheat the oven to 350 degrees.
Microwave the butter (about 40 seconds to just melt it).
With a stand mixer, cream the butter and sugars. Add the vanilla and egg, beat on low speed until just incorporated (10-15 seconds).
Add the flour, baking soda, and salt. Mix until crumbles form. Use your hands to press the crumbles together to make dough.
Use your hands to mix in the crushed Peanut M&Ms.
Divide the dough and roll into 12 large balls.
Place 3 M&Ms into each cookie.
Bake for 9 - 11 minutes. The cookies will still be puffy and not look fully cooked. When you see the slightest bit of brown, pull them out and let them cool on the pan.