Soft M&M Chocolate Chip Cookies

INGREDIENTS:

3/4 cup unsalted butter, softened (1 1/2 sticks)
3/4 cup light brown sugar, packed
1/4 cup granulated sugar
1 large egg
1 teaspoon vanilla extract
2 cups all-purpose flour
1/4 cup instant vanilla pudding mix (not sugar-free and not 'cook & serve'), OR use 2 teaspoons cornstarch in place of pudding mix
1 teaspoon baking soda
pinch salt, optional and to taste
3/4 cup semi-sweet chocolate chips
3/4 cup milk chocolate M&M's mixed into the dough + about 1/2 cup for placing on top of dough mounds

DIRECTIONS:

1. To the bowl of a stand mixer fitted with the paddle attachment (or large mixing bowl and electric mixer) combine the butter, sugars, egg, vanilla, and beat on medium-high speed until creamed and well combined, about 4 minutes.
2. Stop, scrape down the sides of the bowl, and add the flour, pudding mix (or cornstarch), baking soda, optional salt, and beat on low speed until just combined, about 1 minute.
3. Stop, scrape down the sides of the bowl, and add the chocolate chips, 3/4 cup M&Ms, and beat on low speed until just combined, about 30 seconds.
4. Using a large cookie scoop, 1/4-cup measure, or your hands, form 15 equal-sized mounds of dough, roll into balls, and flatten slightly.
5. Add about 1 tablespoon M&Ms to the top of each dough mound.
6. Place mounds on a large plate or tray, cover with plasticwrap, and refrigerate for at least 2 hours, up to 5 days. Do not bake with unchilled dough because cookies will bake thinner, flatter, and be more prone to spreading.
7. Preheat oven to 350F, line a baking sheet with a Silpat or spray with cooking spray. Place dough mounds on baking sheet, spaced at least 2 inches apart (I bake 8 cookies per sheet) and bake for 11 minutes, or until edges have set and tops are just set, even if slightly undercooked, pale, and glossy in the center. Cookies firm up as they cool. Allow cookies to cool on baking sheet for about 10 minutes before serving. I let them cool on the baking sheet and don't use a rack.
8. Cookies will keep airtight at room temperature for up to 1 week or in the freezer for up to 6 months. Alternatively, unbaked cookie dough can be stored in an airtight container in the refrigerator for up to 5 days, so consider baking only as many cookies as desired and save the remaining dough to be baked in the future when desired.