Adjust the oven rack to middle position. Preheat the oven to 350 F.
Grease an 8X8-inch metal pan. Set aside.
Whisk the melted butter and sugars in a large bowl until combined.
Add the egg and vanilla and mix well.
Using a rubber spatula, fold the flour, salt, and baking soda into the butter mixture until just combined; do not overmix.
Fold in the M&Ms and chocolate chips.
Pour the batter into the prepared pan, smoothing the top with the spatula.
Bake until the top of the bars is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan, about 15-25 minutes.
Cool on a wire rack to room temperature.
Butter: I like a good salty/sweetness to my chocolate chip cookies, so I use salted butter along with the salt. If you like your cookies on the sweeter side, use unsalted butter or omit the salt in these.
Melting the butter: Just barely melting the butter helps to speed things along. It takes about 40 seconds in my microwave to melt the butter without getting it too hot.
Flour: To keep these bars soft and chewy, be sure to properly measure the flour. First, sift or stir the flour to break it up. Then lightly spoon it into the measuring cup and level. You don't want to pack the flour or the bars will be heavy and dry.
Salt: Or use ½ teaspoon of kosher salt if you prefer.
M&M's: I use regular chocolate M&M's. Feel free to use a seasonal flavor if you prefer.
Chocolate chips: I like to use mini chips because it gives little bits of chocolate throughout the bars. You can use regular chocolate chips if you prefer.
Mixing the dough: I recommend mixing this dough by hand. It's easy to do, and it helps to prevent overworking the dough.