Preheat your oven to 350°F (175°C) to ensure even baking and perfect golden cookies.
Line two baking sheets with parchment paper to prevent sticking and for easy cleanup.
In a medium bowl, whisk together the flour, fine sea salt, baking soda, and cream of tartar to evenly distribute the leavening agents.
In a large mixing bowl, beat the unsalted butter and vegetable shortening with an electric mixer on medium speed until light and fluffy, about 2-3 minutes.
Gradually add granulated sugar and continue beating until well combined. Then add the whole egg, egg yolk, and vanilla extract, mixing until smooth.
Slowly add the dry ingredients to the wet mixture on low speed, mixing just until combined to avoid tough cookies.
Gently fold 1 cup of milk chocolate M&Ms into the dough with a spatula, ensuring even distribution.
Use a cookie scoop or tablespoon to portion the dough, then roll each piece into a ball and lightly coat in granulated sugar.
Arrange the dough balls on the prepared baking sheets, spacing them to allow spreading.
Press a few additional M&Ms on top of each cookie for a colorful finish.
Bake for 10-12 minutes until edges are lightly golden; centers may look slightly underbaked but will set as they cool.
Let cookies cool on the baking sheets for a few minutes, then transfer to a wire rack to cool completely before enjoying.