Preheat oven to 350°F and adjust baking rack to middle position. Line 2-3 large baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, mix the granulated sugar and brown sugar together on low until just combined.
1 cup granulated sugar, 1 cup dark brown sugar
Add the shortening and butter, turn mixer to medium speed, and beat until lighter in color and smooth, about 3 minutes.
⅔ cup vegetable shortening, ⅔ cup unsalted butter
Add the vanilla and beat on medium speed for 30 seconds.
2 teaspoons pure vanilla extract
Add eggs one at a time, mixing just until incorporated after each addition. Turn off the mixer.
2 large eggs
In a large mixing bowl, whisk the flour, baking soda, coarse salt, and kosher salt together.
2¾ cup all-purpose flour, 1 teaspoon baking soda, ½ teaspoon coarse sea salt, ½ teaspoon kosher salt
Add the flour mixture to the sugar/butter mixture and turn the mixer to low speed. Mix just until flour mixture is incorporated.
Fold in 1¾ cup M&M candies.
2½ cups M&M's candies
If time allows, cover, refrigerate and let the dough rest 1-24 hours. If time doesn’t allow, the cookies can be baked immediately and are still terrific without the rest time.
Drop the dough, using a 2-inch cookie scoop (about 1½ tablespoons), onto the baking sheets about 2 inches apart (12 cookies to a baking sheet).
Press 3-4 additional candies into each cookie.
Bake cookies at 350°F for 9 minutes, or just until the edges of the cookies start to set and are a little firmer to the touch.
Place baking sheets with cookies on a cooling rack for 2 minutes, then transfer cookies directly to the cooling rack.