Soft & Chewy Choco Chip Cookies with M&M's

Soft & Chewy Choco Chip Cookies with M&M's
Recipe from Martha Stewart's Cookies book

2 1/4 cups all-purpose flour
1/2 teaspoon baking soda
1 cup (2 sticks) unsalted butter, room temperature
1/2 cup granulated sugar
1 cup packed light-brown sugar
1 teaspoon salt
2 teaspoons pure vanilla extract
2 large eggs
2 cups (about 12 ounces) semisweet and/or milk chocolate chips
1 1/2 cups plain M&M's (optional)

Preheat oven to 350 degrees. In a small bowl, whisk together the flour and baking soda; set aside. In the bowl of an electric mixer fitted with the paddle attachment, combine the butter with both sugars; beat on medium speed until light and fluffy.
Reduce speed to low; add the salt, vanilla, and eggs. Beat until well mixed, about 1 minute. Add flour mixture; mix until just combined. Stir in the chocolate chips and M&M's.

Drop heaping tablespoon-size balls of dough about 2 inches apart on baking sheets lined with parchment paper.

Bake until cookies are golden around the edges, but still soft in the center, 8 to 10 minutes. Remove from oven, and let cool on baking sheet 1 to 2 minutes. Transfer to a wire rack, and let cool completely.