Clean and dry quart sized mason jar. Using a funnel or spoon add the flour, baking soda and salt. Tap on counter to level. Then spoon in the brown sugar and press down the edges with spoon. Next add the sugar. Then add the chocolate chips and M&M’s. Seal jar.
When getting ready to bake preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.
In a large bowl beat the butter, egg and vanilla. Then dump the jar ingredients and stir until combined. Use hands if needed to combine mixture. Refrigerate for at least 15 minutes for thicker cookies. Scoop the dough onto the pan using a large cookie scoop (or about 2 Tablespoons). Bake for 8-10 minutes or until the edges are slightly crisp, yet still soft in the middle. Remove from pan to cooling rack.
Sealed jar will stay fresh for 1-2 months. Go here for my original M&M cookie recipe.