In a large bowl, use an electric mixer to cream together the butter, brown sugar, and granulated sugar until they are light in color and fluffy in texture.

Add the eggs and vanilla extract to the butter and sugar mixture. Mix again on low speed until everything is combined well.

Add the flour, cornstarch, baking soda, and salt to the bowl. Then mix on low speed until no dry patches remain.

Gently fold in the pretzels, M&Ms, and both types of chocolate chips.

Cover and refrigerate for at least 45 minutes to 1 hour.

Line two large baking sheets with parchment paper, and preheat the oven to 350°F.

Use a 2-tablespoon cookie scoop to measure the dough onto the prepared baking sheets spaced about 2 inches apart. Gently press the mounds of dough flat on the top, and add extra M&Ms, chocolate chips, and pretzels if desired.

Bake for 11 to 15 minutes. The cookies are ready when the edges are brown and the tops are golden brown but still soft in the center.

Let the cookies sit on the baking sheets for 5 minutes before putting them on a wire rack to cool.

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