Preheat your oven to 350°F (175°C). Line 2 large baking sheets with parchment paper or silicone baking mats.

In a large mixing bowl, using a stand mixer with a paddle attachment or a hand mixer, beat the softened butter, brown sugar, and granulated sugar together on medium speed for 2–3 minutes, until light and fluffy.

Add the eggs one at a time, mixing on low after each until just combined. Scrape down the bowl, then mix in the vanilla extract until smooth. Do not overmix.

In a separate medium bowl, whisk together the flour, rolled oats, baking soda, and salt until evenly combined.

Add the dry ingredients to the wet ingredients in two additions, mixing on low just until no streaks of flour remain and a soft dough forms. Avoid overmixing once the flour is added.

Using a spatula or wooden spoon, gently fold in the chocolate chips, butterscotch chips, toffee bits, chopped pretzels, nuts, crushed potato chips, and M&M’s (if using). The dough should be very chunky but still hold together; if it seems too dry or crumbly, add 1–2 teaspoons of milk and gently work it in.

Cover the bowl and chill the dough in the refrigerator for at least 30 minutes and up to 48 hours. Chilling helps the flour hydrate and keeps the cookies thick and chewy instead of spreading too much.

Use a medium cookie scoop (about 2 tablespoons of dough) to portion the dough onto the prepared baking sheets, spacing about 2 inches apart. Gently press a few extra mix-ins onto the tops for a bakery-style look, then sprinkle lightly with flaky sea salt.

Bake one sheet at a time for 10–12 minutes, or until the edges are set and lightly golden while the centers still look soft and slightly underbaked. For crisper cookies, bake 12–13 minutes, watching closely.

Let the cookies cool on the baking sheet for 5 minutes to finish setting, then transfer to a wire rack to cool completely. The centers will firm up as they cool.